SI-7818 Cast our way out of extended existential angst

`substituteSymbols` is not sophisticated enough to
operate on `TypeSkolem`-s which are based on one of the
"from" symbols.

The pertinant usage of `substituteSymbols` for this bug in
in `Extender`. Recapping on that transform:

    // orig
    class C[T](...) extends AnyVal { def foo[U] = <rhs> }

    // transform
    class C[T] extends AnyVal { ... }
    object C  { def foo$extension[T', U'] = <rhs'> }

Where `<rhs'>` has been subtituted with, among other things,
`[T, U] ~> [T', U']`.

In this case our expected type contains a new type parameter
(of the extension method), whereas the type of the RHS contains
an existential skolem still pinned to the corresponding class type
parameter.

    tree.tpe         = Observable1#7037[_$1#12344]
    <_$1#12344>.info =  <: T#7040
    pt               = Observable1#7037[T#15644]

The limitation of substution is lamented in the comments
of `adaptMismatchedSkolems`, which faces the harder version of
the issue where the skolems are in the expected type.

But, we're in the "easy" case with the skolems in the tree's type;
we can cast our way out of the problem.

See also f335e44 / ed915c5.

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

@@ -231,9 +231,14 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
               .substituteThis(origThis, extensionThis)
               .changeOwner(origMeth -> extensionMeth)
           )
+          val castBody =
+            if (extensionBody.tpe <:< extensionMono.finalResultType)
+              extensionBody
+            else
+              gen.mkCastPreservingAnnotations(extensionBody, extensionMono.finalResultType) // SI-7818 e.g. mismatched existential skolems
 
           // Record the extension method ( FIXME: because... ? )
-          extensionDefs(companion) += atPos(tree.pos)(DefDef(extensionMeth, extensionBody))
+          extensionDefs(companion) += atPos(tree.pos)(DefDef(extensionMeth, castBody))
 
           // These three lines are assembling Foo.bar$extension[T1, T2, ...]($this)
           // which leaves the actual argument application for extensionCall.

test/files/pos/t7818.scala

@@ -0,0 +1,10 @@
+class Observable1[+T](val asJava: JObservable[_ <: T]) extends AnyVal {
+  private def foo[X](a: JObservable[X]): JObservable[X] = ???
+  // was generating a type error as the type of the RHS included an existential
+  // skolem based on the class type parameter `T`, which did not conform
+  // to the typer parameter of the extension method into which the RHS is
+  // transplanted.
+  def synchronize: Observable1[T] = new Observable1(foo(asJava))
+}
+
+class JObservable[T]