[Clang] Dispatch default overloads of TemplateArgumentVisitor to the implementation

This fixes an issue where overriding
`clang::ConstTemplateArgumentVisitor::VisitTemplateArgument` in an
implementation visitor class did not have the desired effect: the
overload was not invoked when one of the visitor methods (e.g.
`VisitDeclarationArgument`) is not implemented, instead it dispatched to
`clang::ConstTemplateArgumentVisitor::VisitTemplateArgument` itself and
always returned a default-initialized result.

This makes `TemplateArgumentVisitor` and `ConstTemplateArgumentVisitor`
follow the implicit convention that is followed elsewhere in Clang AST,
in `RecursiveASTVisitor` and `TypeVisitor`.

Author: egorzhdan

clang/include/clang/AST/TemplateArgumentVisitor.h

@@ -52,7 +52,8 @@ class Base {
 #define VISIT_METHOD(CATEGORY)                                                 \
   RetTy Visit##CATEGORY##TemplateArgument(REF(TemplateArgument) TA,            \
                                           ParamTys... P) {                     \
-    return VisitTemplateArgument(TA, std::forward<ParamTys>(P)...);            \
+    return static_cast<ImplClass *>(this)->VisitTemplateArgument(              \
+        TA, std::forward<ParamTys>(P)...);                                     \
   }
 
   VISIT_METHOD(Null);