Allow the parser to detect invalid DeclSpec's. This fixes http://llvm.org/bugs/show_bug.cgi?id=1987.

This commit only "guards" the call to ParseDeclarationSpecifiers() in ParseDeclarationOrFunctionDefinition(). 

We could consider guarding all calls, however this is a bit radical (since it effectively stops parsing the declaration once we have a bad declspec). Will discuss with Chris tomorrow.

llvm-svn: 46984