[Clang] Fix crash in isCXXDeclarationSpecifier when attempting to annotate template name

When attempting to decide if in C++17 a type template for class template
argument deduction and the code is ill-formed the condition to break is
checking the current token is an identifier when it should be checking
if the next token is not ::.

This fixes: https://github.com/llvm/llvm-project/issues/57495
https://github.com/llvm/llvm-project/issues/63052

Differential Revision: https://reviews.llvm.org/D134334