[Sema] Don't emit the -Wstrict-prototypes warning for variadic functions.

rdar://problem/33251668

Reviewers: arphaman, ahatanak

Reviewed By: arphaman

Subscribers: ptitei, cfe-commits

Differential Revision: https://reviews.llvm.org/D41528


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@321660 91177308-0d34-0410-b5e6-96231b3b80d8