[C++20] [Modules] [Serialization] Don't write comments to BMI for C++20 Named Modules

This patch forbids to write comment to BMIs for C++20 Named Modules.
Originally I thought this was helpful for language services like clangd.
But I found clangd don't want the BMI to contain comments actually. So
it is meaningless for C++20 Named Modules to keep such comments in
their BMI.

It is simple to enable this when someday we found we want this actually.