Potential UB in diagnostics for LTO mode.

[bader]

llvm/clang/lib/Driver/ToolChains/Clang.cpp

Lines 6000 to 6023 in 2705b67

	if ((IsDeviceOffloadAction &&
	!JA.isDeviceOffloading(Action::OFK_OpenMP) && !Triple.isAMDGPU() &&
	!IsUsingOffloadNewDriver) ||
	(JA.isDeviceOffloading(Action::OFK_SYCL) && !IsSYCLLTOSupported)) {
	D.Diag(diag::err_drv_unsupported_opt_for_target)
	<< Args.getLastArg(options::OPT_foffload_lto,
	options::OPT_foffload_lto_EQ)
	->getAsString(Args)
	<< Triple.getTriple();
	} else if (Triple.isNVPTX() && !IsRDCMode &&
	JA.isDeviceOffloading(Action::OFK_Cuda)) {
	D.Diag(diag::err_drv_unsupported_opt_for_language_mode)
	<< Args.getLastArg(options::OPT_foffload_lto,
	options::OPT_foffload_lto_EQ)
	->getAsString(Args)
	<< "-fno-gpu-rdc";
	} else if (JA.isDeviceOffloading(Action::OFK_SYCL) &&
	IsDeviceCodeSplitDisabled && LTOMode == LTOK_Thin) {
	D.Diag(diag::err_drv_sycl_thinlto_split_off)
	<< SYCLSplitMode->getAsString(Args)
	<< Args.getLastArg(options::OPT_foffload_lto,
	options::OPT_foffload_lto_EQ)
	->getAsString(Args);
	} else {

The pattern used in this code doesn't the pointer returned by Args.getLastArg(options::OPT_foffload_lto, options::OPT_foffload_lto_EQ) before dereferencing. This can be source of UB.