[DebugInfo] Emit DW_TAG_enumeration_type for referenced global enumerator.

This essentially reverts changes from r361400 while keeping behavior for
CodeView.

Reviewers: akhuang, rnk, probinson

Reviewed by: rnk

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67141

llvm-svn: 370981