SD-167 Fine tuning constructor pattern translation

  - Avoid calling NoSymbol.owner when checking whether we're
    dealing with a case class constructor pattern or a general
    extractor. Tested manually with the test case in the ticket,
    no more output is produced under `-Xdev`.
  - Be more conservative about the conversion to a case class
    pattern: rather than looking just at the type of the pattern
    tree, also look at the tree itself to ensure its safe to
    elide. This change is analagous to SI-4859, which restricted
    rewrites of case apply calls to case constructors.

I've manually tested that case class patterns are still efficiently
translated:

```
object Test {
  def main(args: Array[String]) {
    Some(1) match { case Some(x) => }
  }
}

```

```
% qscalac -Xprint:patmat sandbox/test.scala
[[syntax trees at end of                    patmat]] // test.scala
package <empty> {
  object Test extends scala.AnyRef {
    def <init>(): Test.type = {
      Test.super.<init>();
      ()
    };
    def main(args: Array[String]): Unit = {
      case <synthetic> val x1: Some[Int] = scala.Some.apply[Int](1);
      case4(){
        if (x1.ne(null))
          matchEnd3(())
        else
          case5()
      };
      case5(){
        matchEnd3(throw new MatchError(x1))
      };
      matchEnd3(x: Unit){
        x
      }
    }
  }
}
```

2 files changed, 9 insertions, 0 deletions

diff --git a/test/files/run/sd167.check b/test/files/run/sd167.check
new file mode 100644
index 0000000000..587be6b4c3
--- /dev/null
+++ b/test/files/run/sd167.check
@@ -0,0 +1 @@
+x
diff --git a/test/files/run/sd167.scala b/test/files/run/sd167.scala
new file mode 100644
index 0000000000..5095e772ad
--- /dev/null
+++ b/test/files/run/sd167.scala
@@ -0,0 +1,8 @@
+object Test {
+  implicit class ToExtractor(val s: StringContext) {
+    def x = {println("x"); Some }
+  }
+  def main(args: Array[String]) {
+    Some(1) match { case x"${a}" => }  // used to convert to `case Some(a) =>` and omit side effects
+  }
+}