[clang] [C++20] [Modules] Convert '-fexperimental-modules-reduced-bmi' to '-fmodules-reduced-bmi' (PR #114382)

[Chuanqi Xu via cfe-commits]

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@@ -556,6 +556,14 @@ def err_test_module_file_extension_format : Error<
 def err_drv_module_output_with_multiple_arch : Error<
   "option '-fmodule-output' can't be used with multiple arch options">;
 
+def warn_drv_module_reduced_bmi_not_enabled : Warning<
+  "reduced BMI is expected to be enabled by default in Clang 21. It is encouraged to "
+  "enable it ahead of time to avoid potential breaking change. You can enable it "
+  "by offering '-fmodules-reduced-bmi' option in one phase compilation model (e.g., CMake). "
+  "Or if your build system support two phase compilation model, please contact the "
+  "build system authors to support reduced BMI and turn off the warning temporarily">,
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ChuanqiXu9 wrote:

Done. I feel the new wording is pretty cool and fine enough : )

https://github.com/llvm/llvm-project/pull/114382