[OPENMP50]Emit warnings if the functions was defined/used before marked

[OPENMP50]Emit warnings if the functions was defined/used before marked
declare variant.

We can use the original function if it was used/emitted already. So,
just use warnings for these cases, not errors.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@373010 91177308-0d34-0410-b5e6-96231b3b80d8