Improve presentation compilation of annotations

A trio of problems were hampering autocompletion of annotations.

First, given that that annotation is written before the annotated
member, it is very common to end parse incomplete code that has a
floating annotation without an anotatee.

The parser was discarding the annotations (ie, the modifiers) and
emitting an `EmptyTree`.

Second, the presetation compiler was only looking for annotations
in the Modifiers of a member def, but after typechecking annotations
are moved into the symbol.

Third, if an annotation failed to typecheck, it was being discarded
in place of `ErroneousAnnotation`.

This commit:
  - modifies the parser to uses a dummy class- or type-def tree,
    instead of EmptyTree, which can carry the annotations.
  - updates the locator to look in the symbol annotations of the
    modifiers contains no annotations.
  - uses a separate instance of `ErroneousAnnotation` for each
    erroneous annotation, and stores the original tree in its
    `original` tree.

1 files changed, 8 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/Positions.scala b/src/reflect/scala/reflect/internal/Positions.scala
index 4d0e31b037..15d68bcdfe 100644
--- a/src/reflect/scala/reflect/internal/Positions.scala
+++ b/src/reflect/scala/reflect/internal/Positions.scala
@@ -252,7 +252,14 @@ trait Positions extends api.Positions { self: SymbolTable =>
             super.traverse(t)
           } else t match {
             case mdef: MemberDef =>
-              traverseTrees(mdef.mods.annotations)
+              val annTrees = mdef.mods.annotations match {
+                case Nil if mdef.symbol != null =>
+                  // After typechecking, annotations are mvoed from the modifiers
+                  // to the annotation on the symbol of the anotatee.
+                  mdef.symbol.annotations.map(_.original)
+                case anns => anns
+              }
+              traverseTrees(annTrees)
             case _ =>
           }
       }