[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#123444)

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 ValueOrInherited to be nonnull.  Note that isSet
checks to see if ValueOrInherited is nonnull.

Author: kazutakahirata

clang/include/clang/AST/DeclTemplate.h

@@ -367,12 +367,11 @@ class DefaultArgStorage {
     if (!isSet())
       ValueOrInherited = InheritedFrom;
     else if ([[maybe_unused]] auto *D =
-                 ValueOrInherited.template dyn_cast<ParmDecl *>()) {
+                 dyn_cast<ParmDecl *>(ValueOrInherited)) {
       assert(C.isSameDefaultTemplateArgument(D, InheritedFrom));
       ValueOrInherited =
           new (allocateDefaultArgStorageChain(C)) Chain{InheritedFrom, get()};
-    } else if (auto *Inherited =
-                   ValueOrInherited.template dyn_cast<Chain *>()) {
+    } else if (auto *Inherited = dyn_cast<Chain *>(ValueOrInherited)) {
       assert(C.isSameDefaultTemplateArgument(Inherited->PrevDeclWithDefaultArg,
                                              InheritedFrom));
       Inherited->PrevDeclWithDefaultArg = InheritedFrom;