[flang] Use x.empty() instead of llvm::empty(x) (NFC)

I'm planning to deprecate and eventually remove llvm::empty.

Note that no use of llvm::empty requires the ability of llvm::empty to
determine the emptiness from begin/end only.

flang/unittests/Optimizer/Builder/Runtime/CharacterTest.cpp

@@ -84,7 +84,7 @@ void checkCharCompare2(
   auto cmpOp = mlir::dyn_cast<mlir::arith::CmpIOp>(res.getDefiningOp());
   checkCallOp(cmpOp.getLhs().getDefiningOp(), fctName, 4, /*addLocArgs=*/false);
   auto allocas = res.getParentBlock()->getOps<fir::AllocaOp>();
-  EXPECT_TRUE(llvm::empty(allocas));
+  EXPECT_TRUE(allocas.empty());
 }
 
 TEST_F(RuntimeCallTest, genCharCompare2Test) {