[ByteCode] Migrate away from PointerUnion::dyn_cast (NFC) (#123445)

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 D to be nonnull.

Author: kazutakahirata

clang/lib/AST/ByteCode/Program.cpp

@@ -155,7 +155,7 @@ unsigned Program::getOrCreateDummy(const DeclTy &D) {
 
   QualType QT;
   bool IsWeak = false;
-  if (const auto *E = D.dyn_cast<const Expr *>()) {
+  if (const auto *E = dyn_cast<const Expr *>(D)) {
     QT = E->getType();
   } else {
     const ValueDecl *VD = cast<ValueDecl>(cast<const Decl *>(D));