DR101, PR12770: If a function is declared in the same context as a

DR101, PR12770: If a function is declared in the same context as a
using-declaration, and they declare the same function (either because
the using-declaration is in the same namespace as the declaration it
imports, or because they're both extern "C"), they do not conflict.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200897 91177308-0d34-0410-b5e6-96231b3b80d8