[clang] [llvm] [clang][llvm][fatlto] Avoid cloning modules in FatLTO (PR #72180)

[Teresa Johnson via cfe-commits]

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@@ -1861,6 +1861,13 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
     if (Args.hasArg(OPT_funified_lto))
       Opts.PrepareForThinLTO = true;
   }
+  if (Arg *A = Args.getLastArg(options::OPT_ffat_lto_objects,
+                               options::OPT_fno_fat_lto_objects)) {
+    if (!Args.hasArg(OPT_funified_lto))
+      Diags.Report(diag::err_drv_incompatible_options)
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teresajohnson wrote:

It might be less confusing to users if this error message is only given upon an explicit -fno-unified-lto, and diag::err_drv_argument_only_allowed_with is used for the lack of -funified-lto.

Also can you add driver tests to check that we get the expected error(s) in the expected option combinations?

https://github.com/llvm/llvm-project/pull/72180