Add a hasExternalStorageAsWritten helper. No functionality change.

It is possible that some of the current uses of
"getStorageClassAsWritten() == SC_Extern" should use this but I don't know
enough about SC_PrivateExtern to change and test them.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176606 91177308-0d34-0410-b5e6-96231b3b80d8