[AST] 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 P to be nonnull.

Author: kazutakahirata

clang/lib/AST/DeclTemplate.cpp

@@ -1801,8 +1801,7 @@ TemplateParameterList *clang::getReplacedTemplateParameterList(Decl *D) {
   case Decl::Kind::VarTemplateSpecialization: {
     const auto *VTSD = cast<VarTemplateSpecializationDecl>(D);
     auto P = VTSD->getSpecializedTemplateOrPartial();
-    if (const auto *VTPSD =
-            P.dyn_cast<VarTemplatePartialSpecializationDecl *>())
+    if (const auto *VTPSD = dyn_cast<VarTemplatePartialSpecializationDecl *>(P))
       return VTPSD->getTemplateParameters();
     return cast<VarTemplateDecl *>(P)->getTemplateParameters();
   }