[clang] [clang] Implement gcc_struct attribute on Itanium targets (PR #71148)

[Fangrui Song via cfe-commits]

================
@@ -6113,9 +6113,13 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (KernelOrKext && RawTriple.isOSDarwin())
     CmdArgs.push_back("-fforbid-guard-variables");
 
-  if (Args.hasFlag(options::OPT_mms_bitfields, options::OPT_mno_ms_bitfields,
-                   Triple.isWindowsGNUEnvironment())) {
-    CmdArgs.push_back("-mms-bitfields");
+  if (Args.hasArg(options::OPT_mms_bitfields) ||
----------------
MaskRay wrote:

If the LangOption default varies across targets, and you want to forward the driver option to clang -cc1,

```
if (auto *A = Args.getLastArg(..., ...)) {
  if (A->matches(...)) CmdArgs.push_back...
  else CmdArgs.push_back...
}
```

https://github.com/llvm/llvm-project/pull/71148