Don't asume that a static function in an extern "C" block will not be mangled.

Don't asume that a static function in an extern "C" block will not be mangled.
Since functions with internal linkage don't have language linkage, it is valid
to overload them:

extern "C" {
       static int foo();
       static int foo(int);
}

So we mangle them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175120 91177308-0d34-0410-b5e6-96231b3b80d8