[AST] Fix clang RecursiveASTVisitor for definition of XXXTemplateSpecializationDecl

Fixes https://github.com/clangd/clangd/issues/1132
where clangd's semantic highlighting is missing for symbols of a template
specialization definition. It turns out the visitor didn't traverse the
base classes of Class/Var##TemplateSpecializationDecl, i.e.
CXXRecordDecl/VarDecl. This patch adds them back as what is done in
DEF_TRAVERSE_TMPL_PART_SPEC_DECL.

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D126757