Fix enclosing method attribute for classes nested in trait fields

Trait fields are now created as MethodSymbol (no longer TermSymbol).
This symbol shows up in the `originalOwner` chain of a class declared
within the field initializer. This promoted the field getter to
being the enclosing method of the nested class, which it is not
(the EnclosingMethod attribute is a source-level property).

src/compiler/scala/tools/nsc/backend/jvm/BCodeAsmCommon.scala

@@ -162,7 +162,7 @@ final class BCodeAsmCommon[G <: Global](val global: G) {
 
     def enclosingMethod(sym: Symbol): Option[Symbol] = {
       if (sym.isClass || sym == NoSymbol) None
-      else if (sym.isMethod) {
+      else if (sym.isMethod && !sym.isGetter) {
         if (doesNotExist(sym)) None else Some(sym)
       }
       else enclosingMethod(nextEnclosing(sym))