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

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 TransExpr to be nonnull.

Author: kazutakahirata

clang/lib/Sema/TreeTransform.h

@@ -14903,7 +14903,7 @@ TreeTransform<Derived>::TransformExprRequirement(concepts::ExprRequirement *Req)
     TransRetReq.emplace(TPL);
   }
   assert(TransRetReq && "All code paths leading here must set TransRetReq");
-  if (Expr *E = TransExpr.dyn_cast<Expr *>())
+  if (Expr *E = dyn_cast<Expr *>(TransExpr))
     return getDerived().RebuildExprRequirement(E, Req->isSimple(),
                                                Req->getNoexceptLoc(),
                                                std::move(*TransRetReq));