[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#125811)

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 Specialized to be nonnull. Note that if Specialized
were
null, dereferencing Tmpl would trigger a segfault.

Author: kazutakahirata

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -190,7 +190,7 @@ HandleVarTemplateSpec(const VarTemplateSpecializationDecl *VarTemplSpec,
   llvm::PointerUnion<VarTemplateDecl *, VarTemplatePartialSpecializationDecl *>
       Specialized = VarTemplSpec->getSpecializedTemplateOrPartial();
   if (VarTemplatePartialSpecializationDecl *Partial =
-          Specialized.dyn_cast<VarTemplatePartialSpecializationDecl *>()) {
+          dyn_cast<VarTemplatePartialSpecializationDecl *>(Specialized)) {
     if (!SkipForSpecialization)
       Result.addOuterTemplateArguments(
           Partial, VarTemplSpec->getTemplateInstantiationArgs().asArray(),