Fix structural inequivalency of forward EnumDecl

Summary:
Currently we consider one forward declared RecordDecl and another with a
definition equal. We have to do the same in case of enums.

Reviewers: a_sidorin, r.stahl, xazax.hun

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D50444

llvm-svn: 339336