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, 1 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index fc0a0368ab..e017c7d864 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3401,7 +3401,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           // governed by a) the argument types and b) the expected type
           val args1 = typedArgs(args, forArgMode(fun, mode))
           val pts = args1.map(_.tpe.deconst)
-          val clone = fun.symbol.cloneSymbol
+          val clone = fun.symbol.cloneSymbol.withoutAnnotations
           val cloneParams = pts map (pt => clone.newValueParameter(currentUnit.freshTermName()).setInfo(pt))
           val resultType = if (isFullyDefined(pt)) pt else ObjectTpe
           clone.modifyInfo(mt => copyMethodType(mt, cloneParams, resultType))