[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#125457)

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 typeDecl to be nonnull.  Note that
getObjCInterfaceType starts out dereferencing Decl.

Author: kazutakahirata

clang/lib/Sema/SemaDeclObjC.cpp

@@ -1584,7 +1584,7 @@ void SemaObjC::actOnObjCTypeArgsOrProtocolQualifiers(
     const char* prevSpec; // unused
     unsigned diagID; // unused
     QualType type;
-    if (auto *actualTypeDecl = typeDecl.dyn_cast<TypeDecl *>())
+    if (auto *actualTypeDecl = dyn_cast<TypeDecl *>(typeDecl))
       type = Context.getTypeDeclType(actualTypeDecl);
     else
       type = Context.getObjCInterfaceType(cast<ObjCInterfaceDecl *>(typeDecl));