[Sema] Suppress a warning about a forward-declared fixed enum in C mode

As of r343360, we support fixed-enums in C. This lead to some
warnings in project headers where a fixed enum is forward declared
then later defined. In C++, this is fine, the forward declaration is
treated as a complete type even though the definition isn't present.
We use this rule in C too, but still warn about the forward
declaration anyways. This patch suppresses the warning.

rdar://problem/47356469

Differential revision: https://reviews.llvm.org/D56879

llvm-svn: 351595