[NFC] [C++20] [Modules] Refactoring b6c71771 to make it not

dependent on f109b101

Given
https://github.com/llvm/llvm-project/commit/f109b1016801e2b0dbee278f3c517057c0b1d441#commitcomment-113477829,
we need to revert f109b101. So it will be better to make this patch not
dependent on f109b101 as much as possible.