[Utils] Support class template specializations in update_cc_test_checks

ClassTemplateSpecializationDecl not within a ClassTemplateDecl
represents an explicit instatiation of a template and so should be
handled as if it were a normal CXXRecordDecl. Unfortunately, having an
equivalent for FunctionTemplateDecl remains a TODO in ASTDumper's
VisitFunctionTemplateDecl, with all the explicit instantiations just
being emitted inside the FunctionTemplateDecl along with all the other
specializations, meaning we can't easily support explicit function
instantiations in update_cc_test_checks.

Reviewed By: arichardson

Differential Revision: https://reviews.llvm.org/D106243