Remove hasExternalLinkageUncached.

It was being used correctly, but it is a very dangerous API to have around.
Instead, move the logic from the filtering to when we are deciding if we should
link two decls.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179523 91177308-0d34-0410-b5e6-96231b3b80d8