[CodeGen] Migrate away from PointerUnion::dyn_cast (NFC) (#123157)

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 Data to be nonnull.

Author: kazutakahirata

clang/lib/CodeGen/ConstantInitBuilder.cpp

@@ -29,7 +29,7 @@ llvm::Type *ConstantInitFuture::getType() const {
 
 void ConstantInitFuture::abandon() {
   assert(Data && "abandoning null future");
-  if (auto builder = Data.dyn_cast<ConstantInitBuilderBase*>()) {
+  if (auto *builder = dyn_cast<ConstantInitBuilderBase *>(Data)) {
     builder->abandon(0);
   }
   Data = nullptr;