[C++20] [Modules] [Reduced BMI] Avoid force writing static declarations

within module purview

Close https://github.com/llvm/llvm-project/issues/90259

Technically, the static declarations shouldn't be leaked from the module
interface, otherwise it is an illegal program according to the spec. So
we can get rid of the static declarations from the reduced BMI
technically. Then we can close the above issue.

However, there are too many `static inline` codes in existing headers.
So it will be a pretty big breaking change if we do this globally.