[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#123890)

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 TemplateOrSpecialization to be nonnull.

Author: kazutakahirata

clang/lib/AST/Decl.cpp

@@ -4003,7 +4003,7 @@ const IdentifierInfo *FunctionDecl::getLiteralIdentifier() const {
 FunctionDecl::TemplatedKind FunctionDecl::getTemplatedKind() const {
   if (TemplateOrSpecialization.isNull())
     return TK_NonTemplate;
-  if (const auto *ND = TemplateOrSpecialization.dyn_cast<NamedDecl *>()) {
+  if (const auto *ND = dyn_cast<NamedDecl *>(TemplateOrSpecialization)) {
     if (isa<FunctionDecl>(ND))
       return TK_DependentNonTemplate;
     assert(isa<FunctionTemplateDecl>(ND) &&