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

Reverting r208106 to reapply r208065 with a fix for the regression. The
issue was that the enum tried to be built even if the declaration hadn't
been constructed for debug info - presenting problems for enum templates
and typedefs of enums with names for linkage purposes.

Original commit message:

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.