Update for llvm api change.

Now that llvm cannot represent alias cycles, we have to diagnose erros just
before trying to close the cycle. This degrades the errors a bit. The real
solution is what it was before: if we want to provide good errors for these
cases, we have to be able to find a clang level decl given a mangled name
and produce the error from Sema.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209008 91177308-0d34-0410-b5e6-96231b3b80d8