Add a hasExternalLinkage helper. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176607 91177308-0d34-0410-b5e6-96231b3b80d8
Don't duplicate function names. Thanks to Jordan Rose for pointing it out.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176609 91177308-0d34-0410-b5e6-96231b3b80d8