DebugInfo: Don't prefer declarations over definitions in -flimit-debug-info in C

Without an ODR, the -flimit-debug-info debug info size optimization of
emitting declarations for fully defined types when only a declaration is
needed by the TU, is incorrect. Emit the full definition whenever it's
available in non-C++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187611 91177308-0d34-0410-b5e6-96231b3b80d8