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
```

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))

src/reflect/scala/reflect/internal/Definitions.scala

@@ -537,7 +537,8 @@ trait Definitions extends api.StandardDefinitions {
     lazy val ScalaSignatureAnnotation = requiredClass[scala.reflect.ScalaSignature]
     lazy val ScalaLongSignatureAnnotation = requiredClass[scala.reflect.ScalaLongSignature]
 
-    lazy val MethodHandle = getClassIfDefined("java.lang.invoke.MethodHandle")
+    lazy val MethodHandleClass = getClassIfDefined("java.lang.invoke.MethodHandle")
+    lazy val VarHandleClass = getClassIfDefined("java.lang.invoke.VarHandle")
 
     // Option classes
     lazy val OptionClass: ClassSymbol   = requiredClass[Option[_]]
@@ -1567,9 +1568,12 @@ trait Definitions extends api.StandardDefinitions {
 
       lazy val PartialManifestClass  = getTypeMember(ReflectPackage, tpnme.ClassManifest)
       lazy val ManifestSymbols = Set[Symbol](PartialManifestClass, FullManifestClass, OptManifestClass)
+      private lazy val PolymorphicSignatureClass = MethodHandleClass.companionModule.info.decl(TypeName("PolymorphicSignature"))
 
-      def isPolymorphicSignature(sym: Symbol) = PolySigMethods(sym)
-      private lazy val PolySigMethods: Set[Symbol] = Set[Symbol](MethodHandle.info.decl(sn.Invoke), MethodHandle.info.decl(sn.InvokeExact)).filter(_.exists)
+      def isPolymorphicSignature(sym: Symbol) = sym != null && sym.isJavaDefined && {
+        val owner = sym.safeOwner
+        (owner == MethodHandleClass || owner == VarHandleClass) && sym.hasAnnotation(PolymorphicSignatureClass)
+      }
 
       lazy val Scala_Java8_CompatPackage = rootMirror.getPackageIfDefined("scala.runtime.java8")
     }

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