Don't special case one line extern "C" decls.

We already avoided warning for

extern "C" const char *Version_string = "2.9";

now we also don't produce any warnings for

extern "C" {
  extern const char *Version_string2 = "2.9";
}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178333 91177308-0d34-0410-b5e6-96231b3b80d8