[C++20] [Modules] Introduce ForceCheckCXX20ModulesInputFiles options for

C++20 modules

Previously, we banned the check for input files from C++20 modules since
we thought the BMI from C++20 modules should be a standalone artifact.

However, during the recent experiment with clangd for modules, I find
it is necessary to tell whether or not a BMI is out-of-date by checking the
input files especially for language servers.

So this patch brings a header search option
ForceCheckCXX20ModulesInputFiles to allow the tools (concretly, clangd)
to check the input files from BMI.