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

[Chuanqi Xu via cfe-commits]

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@@ -691,7 +691,7 @@ ensure it is reachable, e.g. ``using N::g;``.
 Support for Reduced BMIs is still experimental, but it may become the default
 in the future. The expected roadmap for Reduced BMIs as of Clang 19.x is:
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ChuanqiXu9 wrote:

Done as suggested.

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