DebugInfo: Emit the definition of enums when the definition preceeds the declaration and initial use.

This regressed a little further 208055 though it was already a little
broken.

While the requiresCompleteType optimization should be implemented here.
Future (possibly near future) work.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208065 91177308-0d34-0410-b5e6-96231b3b80d8