[C++20] [Modules] Allow -fmodule-file=<module-name>=<BMI-Path> for...

[C++20] [Modules] Allow -fmodule-file=<module-name>=<BMI-Path> for implementation unit and document the behavior

Close https://github.com/llvm/llvm-project/issues/57293.

Previsouly we can't use `-fmodule-file=<module-name>=<BMI-Path>` for
implementation units, it is a bug. Also the behavior of the above option
is not tested nor documented for C++20 Modules. This patch addresses the
2 problems.