[AST] Fix loss of enum forward decl from decl context

For example, given:

  enum __attribute__((deprecated)) T *p;

-ast-print produced:

  enum T *p;

The attribute was lost because the enum forward decl was lost.

Another example is the loss of enum forward decls from C++ namespaces
(in MS compatibility mode).

The trouble was that the EnumDecl node was suppressed, as revealed by
-ast-dump.  The suppression of the EnumDecl was intentional in
r116122, but I don't understand why.  The suppression isn't needed for
the test suite to behave.

Reviewed by: rsmith

Differential Revision: https://reviews.llvm.org/D46846

llvm-svn: 333574