Compiler support for JEP-193 VarHandle polymorphic signatures

VarHandles bring a host of new "polymorphic signature" methods to
the Java 9 standard library. This commit updates the way we detect
such methods to look at the absense/presense of the
`PolymorphicSignature` annotation, so as to include these (and any
future additions.)

When we see applications of such methods, we disable adaptation
of argument and return types.

Tested manually with JDK9-ea:

```
Welcome to Scala 2.12.2-20161208-165912-3de1c0c (Java HotSpot(TM) 64-Bit Server VM, Java 9-ea).
Type in expressions for evaluation. Or try :help.

scala> import java.lang.invoke._, scala.runtime.IntRef
import java.lang.invoke._
import scala.runtime.IntRef

scala> val varHandle = MethodHandles.lookup().in(classOf[IntRef]).findVarHandle(classOf[IntRef], "elem", classOf[Int]);
varHandle: java.lang.invoke.VarHandle = java.lang.invoke.VarHandleInts$FieldInstanceReadWrite@7112ce6

scala> varHandle.getAndSet(ref, 1): Int
res5: Int = 0

scala> varHandle.getAndSet(ref, 1): Int
res6: Int = 1

```

Inspecting bytecode shows the absense of box/unboxing:

```
object Test {
  import java.lang.invoke._, scala.runtime.IntRef
  val varHandle = MethodHandles.lookup().in(classOf[IntRef]).findVarHandle(classOf[IntRef], "elem", classOf[Int]);

  def main(args: Array[String]): Unit = {
    val i : Int = varHandle.getAndSet(IntRef.zero, 1)
  }
}

```

```
  public void main(java.lang.String[]);
    Code:
       0: aload_0
       1: invokevirtual #28                 // Method varHandle:()Ljava/lang/invoke/VarHandle;
       4: invokestatic  #34                 // Method scala/runtime/IntRef.zero:()Lscala/runtime/IntRef;
       7: iconst_1
       8: invokevirtual #40                 // Method java/lang/invoke/VarHandle.getAndSet:(Lscala/runtime/IntRef;I)I
      11: istore_2
      12: return
```

1 files changed, 2 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
index d5d62b2203..95d6662d14 100644
--- a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
+++ b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
@@ -323,7 +323,8 @@ trait JavaUniverseForce { self: runtime.JavaUniverse  =>
     definitions.QuasiquoteClass_api_unapply
     definitions.ScalaSignatureAnnotation
     definitions.ScalaLongSignatureAnnotation
-    definitions.MethodHandle
+    definitions.MethodHandleClass
+    definitions.VarHandleClass
     definitions.OptionClass
     definitions.OptionModule
     definitions.SomeClass