[clang] [C++20] [Modules] Introduce -fexperimental-modules-reduced-bmi (PR #85050)

[Chuanqi Xu via cfe-commits]

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@@ -3017,6 +3017,7 @@ defm prebuilt_implicit_modules : BoolFOption<"prebuilt-implicit-modules",
 
 def fmodule_output_EQ : Joined<["-"], "fmodule-output=">,
   Flags<[NoXarchOption]>, Visibility<[ClangOption, CC1Option]>,
+  MarshallingInfoString<FrontendOpts<"ModuleOutputPath">>,
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ChuanqiXu9 wrote:

Since the flag was only used in drivers. So we didn't need a variable. But after this patch, we need a variable. If we separate this out, we may get a unused variable in that patch. I feel it is not so good.

https://github.com/llvm/llvm-project/pull/85050