[Backport] Modify ArrayBuilder and WrappedArrayBuilder to be reusable

As they're reusable in 2.12.x with this change[1], it'd be useful
to make them reusable in 2.11.x.

[1] https://github.com/scala/scala/commit/6eaae1b969b68ed3dc65a40613a8168b09246256

With this change, not only are they reusable but also we can avoid
mutation of previously created arrays.

Behaviour(Problem):
Actual behaviour before this modification is as follows;

<ArrayBuilder>
```
scala> import scala.collection.mutable.ArrayBuilder
import scala.collection.mutable.ArrayBuilder

scala> val builder = new ArrayBuilder.ofInt
builder: scala.collection.mutable.ArrayBuilder.ofInt = ArrayBuilder.ofInt

scala> builder ++= Vector.range(1, 17)
res0: builder.type = ArrayBuilder.ofInt

scala> val arr = builder.result()
arr: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)

scala> builder.clear()

scala> builder += 100
res2: builder.type = ArrayBuilder.ofInt

scala> val arr2 = builder.result()
arr2: Array[Int] = Array(100)

scala> arr
res3: Array[Int] = Array(100, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
// arr should be Array(1, .., 16) but was unexpectedly mutated by `+=` operation
```

`arr` was mutated as follows;
1. `result` & `clear`
    - `arr = elems`
    - `size = 0`
2. `+=(100)`
    - `ensureSize(0 + 1)`
      => `capacity < size || capacity == 0` is `false` as `capacity == 16`
         and `size == 1`
    - `elems(0) = 100` this is where `arr(0) = 100` was done because we
       did not reallocate a new array for `elems` when calling `ensureSize`,
       which should have happened.
    - `size = 1`
3. `result`
    - `mkArray(1)` gives us `arr2 = Array(100)`

<WrappedArrayBuilder>
We can observe almost the same mutation behaviour of ArrayBuilder.

```
scala> import scala.collection.mutable.WrappedArray
import scala.collection.mutable.WrappedArray

scala> import scala.collection.mutable.WrappedArrayBuilder
import scala.collection.mutable.WrappedArrayBuilder

scala> import scala.reflect.ClassTag
import scala.reflect.ClassTag

scala> val builder = new WrappedArrayBuilder(ClassTag.Int)
builder: scala.collection.mutable.WrappedArrayBuilder[Int] = scala.collection.mutable.WrappedArrayBuilder@56cbfb61

scala> builder ++= Vector.range(1, 17)
res0: builder.type = scala.collection.mutable.WrappedArrayBuilder@56cbfb61

scala> builder.result()
res1: scala.collection.mutable.WrappedArray[Int] = WrappedArray(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)

scala> builder.clear()

scala> builder += 100
res3: builder.type = scala.collection.mutable.WrappedArrayBuilder@56cbfb61

scala> res1
res4: scala.collection.mutable.WrappedArray[Int] = WrappedArray(100, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)

scala> builder.result()
res5: scala.collection.mutable.WrappedArray[Int] = WrappedArray(100)
```

Solution:
We should reset `capacity` to `0` when calling `result` so that
`ensureSize(1)` calls `resize(16)`, which satisfies the property of Builder
reusability. Besides mutation of previously created arrays does not happen.

2 files changed, 44 insertions, 11 deletions

diff --git a/src/library/scala/collection/mutable/ArrayBuilder.scala b/src/library/scala/collection/mutable/ArrayBuilder.scala
index 6e53824cbe..f4ca27dcba 100644
--- a/src/library/scala/collection/mutable/ArrayBuilder.scala
+++ b/src/library/scala/collection/mutable/ArrayBuilder.scala
@@ -104,7 +104,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -169,7 +172,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -234,7 +240,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -299,7 +308,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -364,7 +376,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -429,7 +444,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -494,7 +512,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -559,7 +580,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -623,7 +647,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
@@ -688,7 +715,10 @@ object ArrayBuilder {
     }
 
     def result() = {
-      if (capacity != 0 && capacity == size) elems
+      if (capacity != 0 && capacity == size) {
+        capacity = 0
+        elems
+      }
       else mkArray(size)
     }
 
diff --git a/src/library/scala/collection/mutable/WrappedArrayBuilder.scala b/src/library/scala/collection/mutable/WrappedArrayBuilder.scala
index bfe95a11ab..5781ec91be 100644
--- a/src/library/scala/collection/mutable/WrappedArrayBuilder.scala
+++ b/src/library/scala/collection/mutable/WrappedArrayBuilder.scala
@@ -78,7 +78,10 @@ class WrappedArrayBuilder[A](tag: ClassTag[A]) extends Builder[A, WrappedArray[A
   }
 
   def result() = {
-    if (capacity != 0 && capacity == size) elems
+    if (capacity != 0 && capacity == size) {
+      capacity = 0
+      elems
+    }
     else mkArray(size)
   }