[fixup][Sema] Allow in C to define tags inside enumerations.

Fix for too aggressive error err_type_defined_in_enum introduced in
r313386. Defining tags inside enumerations is prohibited in C++ but
allowed in C.

Reviewers: aaron.ballman, rnk, doug.gregor

Reviewed By: rnk

Subscribers: alberto_magni, cfe-commits

Differential Revision: https://reviews.llvm.org/D38109

llvm-svn: 313894