[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#125335)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we expect InVectors.front() and P to be nonnull.

Author: kazutakahirata

clang/lib/AST/TemplateName.cpp

@@ -144,7 +144,7 @@ TemplateName::TemplateName(DeducedTemplateStorage *Deduced)
 bool TemplateName::isNull() const { return Storage.isNull(); }
 
 TemplateName::NameKind TemplateName::getKind() const {
-  if (auto *ND = Storage.dyn_cast<Decl *>()) {
+  if (auto *ND = dyn_cast<Decl *>(Storage)) {
     if (isa<UsingShadowDecl>(ND))
       return UsingTemplate;
     assert(isa<TemplateDecl>(ND));