SemaCXX: Don't crash when annotation tokens show up before the tag name

Clang has a hack to accept definitions of structs with tag names which
have the same name as intrinsics.  However, this hack didn't guard
against annotation tokens showing up in the token stream.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224909 91177308-0d34-0410-b5e6-96231b3b80d8