Update for llvm api change.

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