[NFC] [C++20] [Modules] Refactor the warning to '-fmodule-file=<BMIPath>' for C++20 modules

Previous implementation of the warning to use `-fmodule-file=<BMIPath>`
for C++20 Modules is not straightforward and it is problematic in case
we read the BMIPath by writing tools based clang components. This patch
refactors it with a simple and direct style.