Keep SAM body in anonfun method in enclosing class

Rather than in implementation of the abstract method in the
expanded anonymous class.

This leads to more more efficient use of the constant pool,
code shapes more amenable to SAM inlining, and is compatible
with the old behaviour of `-Xexperimental` in Scala 2.11,
which ScalaJS now relies upon.

Manual test:

```
scala> :paste -raw
// Entering paste mode (ctrl-D to finish)

package p1; trait T { val x = 0; def apply(): Any }; class DelambdafyInline { def t: T = (() => "") }

// Exiting paste mode, now interpreting.

scala> :javap -c p1.DelambdafyInline
Compiled from "<pastie>"
public class p1.DelambdafyInline {
  public p1.T t();
    Code:
       0: new           #10                 // class p1/DelambdafyInline$$anonfun$t$1
       3: dup
       4: aload_0
       5: invokespecial #16                 // Method p1/DelambdafyInline$$anonfun$t$1."<init>":(Lp1/DelambdafyInline;)V
       8: areturn

  public final java.lang.Object p1$DelambdafyInline$$$anonfun$1();
    Code:
       0: ldc           #22                 // String
       2: areturn

  public p1.DelambdafyInline();
    Code:
       0: aload_0
       1: invokespecial #25                 // Method java/lang/Object."<init>":()V
       4: return
}

scala> :javap -c p1.DelambdafyInline$$anonfun$t$1
Compiled from "<pastie>"
public final class p1.DelambdafyInline$$anonfun$t$1 implements p1.T,scala.Serializable {
  public static final long serialVersionUID;

  public int x();
    Code:
       0: aload_0
       1: getfield      #25                 // Field x:I
       4: ireturn

  public void p1$T$_setter_$x_$eq(int);
    Code:
       0: aload_0
       1: iload_1
       2: putfield      #25                 // Field x:I
       5: return

  public final java.lang.Object apply();
    Code:
       0: aload_0
       1: getfield      scala#34                 // Field $outer:Lp1/DelambdafyInline;
       4: invokevirtual scala#37                 // Method p1/DelambdafyInline.p1$DelambdafyInline$$$anonfun$1:()Ljava/lang/Object;
       7: areturn

  public p1.DelambdafyInline$$anonfun$t$1(p1.DelambdafyInline);
    Code:
       0: aload_1
       1: ifnonnull     6
       4: aconst_null
       5: athrow
       6: aload_0
       7: aload_1
       8: putfield      scala#34                 // Field $outer:Lp1/DelambdafyInline;
      11: aload_0
      12: invokespecial scala#42                 // Method java/lang/Object."<init>":()V
      15: aload_0
      16: invokespecial scala#45                 // Method p1/T.$init$:()V
      19: return
}

scala> :quit
```

Author: retronym

src/compiler/scala/tools/nsc/transform/UnCurry.scala

@@ -75,9 +75,9 @@ abstract class UnCurry extends InfoTransform
 
     // Expand `Function`s in constructors to class instance creation (SI-6666, SI-8363)
     // We use Java's LambdaMetaFactory (LMF), which requires an interface for the sam's owner
-    private def mustExpandFunction(fun: Function) = forceExpandFunction || {
+    private def mustExpandFunction(fun: Function) = {
       // (TODO: Can't use isInterface, yet, as it hasn't been updated for the new trait encoding)
-      val canUseLambdaMetaFactory = inConstructorFlag == 0 && (fun.attachments.get[SAMFunction] match {
+      val canUseLambdaMetaFactory = (fun.attachments.get[SAMFunction] match {
         case Some(SAMFunction(userDefinedSamTp, sam)) =>
           // LambdaMetaFactory cannot mix in trait members for us, or instantiate classes -- only pure interfaces need apply
           erasure.compilesToPureInterface(erasure.javaErasure(userDefinedSamTp).typeSymbol) &&
@@ -207,8 +207,10 @@ abstract class UnCurry extends InfoTransform
     def transformFunction(fun: Function): Tree =
       // Undo eta expansion for parameterless and nullary methods
       if (fun.vparams.isEmpty && isByNameRef(fun.body)) { noApply += fun.body ; fun.body }
-      else if (mustExpandFunction(fun)) gen.expandFunction(localTyper)(fun, inConstructorFlag)
-      else {
+      else if (forceExpandFunction || inConstructorFlag != 0) {
+        // Expand the function body into an anonymous class
+        gen.expandFunction(localTyper)(fun, inConstructorFlag)
+      } else {
         // method definition with the same arguments, return type, and body as the original lambda
         val liftedMethod = gen.mkLiftedFunctionBodyMethod(localTyper)(fun.symbol.owner, fun)
 
@@ -217,7 +219,11 @@ abstract class UnCurry extends InfoTransform
           gen.mkForwarder(gen.mkAttributedRef(liftedMethod.symbol), (fun.vparams map (_.symbol)) :: Nil)
         ))
 
-        localTyper.typedPos(fun.pos)(Block(liftedMethod, super.transform(newFun)))
+        val typedNewFun = localTyper.typedPos(fun.pos)(Block(liftedMethod, super.transform(newFun)))
+        if (mustExpandFunction(fun)) {
+          val Block(stats, expr : Function) = typedNewFun
+          treeCopy.Block(typedNewFun, stats, gen.expandFunction(localTyper)(expr, inConstructorFlag))
+        } else typedNewFun
       }
 
     def transformArgs(pos: Position, fun: Symbol, args: List[Tree], formals: List[Type]) = {