[CIR] Change AtomicFenceOp's syncscope to OptionalAttr

[FantasqueX]

This PR make CIR's AtomicFenceOp similar to MLIR LLVMIR's FenceOp.

MLIR LLVMIR FenceOp:

clangir/mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

Lines 1925 to 1947 in 0bedc28

	def LLVM_FenceOp : LLVM_Op<"fence">, LLVM_MemOpPatterns {
	let arguments = (ins AtomicOrdering:$ordering,
	OptionalAttr<StrAttr>:$syncscope);
	let assemblyFormat = "(`syncscope` `(` $syncscope^ `)`)? $ordering attr-dict";
	string llvmInstName = "Fence";
	let llvmBuilder = [{
	auto *inst = builder.CreateFence(convertAtomicOrderingToLLVM($ordering));
	}] # setSyncScopeCode;
	string mlirBuilder = [{
	llvm::FenceInst *fenceInst = cast<llvm::FenceInst>(inst);
	$_op = $_builder.create<LLVM::FenceOp>(
	$_location,
	convertAtomicOrderingFromLLVM(fenceInst->getOrdering()),
	getLLVMSyncScope(fenceInst));
	}];
	let builders = [
	LLVM_VoidResultTypeOpBuilder,
	LLVM_ZeroResultOpBuilder,
	OpBuilder<(ins "LLVM::AtomicOrdering":$ordering,
	CArg<"StringRef", "StringRef()">:$syncscope)>
	];
	let hasVerifier = 1;
	}