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

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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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h-vetinari wrote:

It's confusing to update/clarify the plan during the 20.x cycle and then not update the "as of 19.x" (even though the overall shape of the plan didn't change). If you want to keep the "as of 19.x", then we should specify which steps have been done already (see below), though IMO _any_ updates here should also update the "as of".

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