[CodeGen] Migrate away from PointerUnion::dyn_cast (NFC) (#122778)

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 Prototype.P to be nonnull.

Author: kazutakahirata

clang/lib/CodeGen/CGCall.cpp

@@ -4507,7 +4507,7 @@ void CodeGenFunction::EmitCallArgs(
   // First, if a prototype was provided, use those argument types.
   bool IsVariadic = false;
   if (Prototype.P) {
-    const auto *MD = Prototype.P.dyn_cast<const ObjCMethodDecl *>();
+    const auto *MD = dyn_cast<const ObjCMethodDecl *>(Prototype.P);
     if (MD) {
       IsVariadic = MD->isVariadic();
       ExplicitCC = getCallingConventionForDecl(