[Serialization] Migrate away from PointerUnion::dyn_cast (NFC) (#124676)

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 FD->TemplateOrSpecialization to be nonnull.

Author: kazutakahirata

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -4682,8 +4682,8 @@ void ASTDeclReader::UpdateDecl(Decl *D) {
         MSInfo->setPointOfInstantiation(POI);
       } else {
         auto *FD = cast<FunctionDecl>(D);
-        if (auto *FTSInfo = FD->TemplateOrSpecialization
-                    .dyn_cast<FunctionTemplateSpecializationInfo *>())
+        if (auto *FTSInfo = dyn_cast<FunctionTemplateSpecializationInfo *>(
+                FD->TemplateOrSpecialization))
           FTSInfo->setPointOfInstantiation(POI);
         else
           cast<MemberSpecializationInfo *>(FD->TemplateOrSpecialization)