Don't mark 'extern "C" void f(void)' as having extern storage class.

Instead, we check for one line extern "C" context in linkage computation and
when deciding if a variable is a definition.

This hopefully completes the transition to having "as written" semantics for
hasExternalStorage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180258 91177308-0d34-0410-b5e6-96231b3b80d8