Parse: Don't crash when 'typename' shows up in an attribute

isDeclarationSpecifier performs error recovers which jostles the token
stream.  Specifically, TryAnnotateTypeOrScopeToken will end up consuming
a typename token which will confuse the attribute parsing machinery as
we no-longer have something identifier-like.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224903 91177308-0d34-0410-b5e6-96231b3b80d8