[Sema] Migrate away from PointerUnion::dyn_cast (NFC)

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 Stored to be nonnull.

Author: kazutakahirata

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -4661,7 +4661,7 @@ void LocalInstantiationScope::InstantiatedLocal(const Decl *D, Decl *Inst) {
     }
 #endif
     Stored = Inst;
-  } else if (DeclArgumentPack *Pack = Stored.dyn_cast<DeclArgumentPack *>()) {
+  } else if (DeclArgumentPack *Pack = dyn_cast<DeclArgumentPack *>(Stored)) {
     Pack->push_back(cast<VarDecl>(Inst));
   } else {
     assert(cast<Decl *>(Stored) == Inst && "Already instantiated this local");