[libc++][modules] Simplifies C++20 module testing.

The building of the std module has been moved from `params.py` and
`dsl.py` to a `lit.local.cfg` for the entire test suite. In theory this
change allows testing modules in most configurations, except:
- combined with clang modules
- C++ versions that don't support the std module

Currently only C++23 with all parts enabled works.
C++26 is expected to work properly with CMake 3.27. That versions of CMake
knows how to invoke clang using C++26.
The parts disabled modi of libc++ have not been modularized yet.

It still is the goal that in the future CMake will be able to do the work
done in `lit.local.cfg`. Doing this in CMake would require a more mature
libc++ implementation.

Thanks a lot to @ldionne for giving hints how to enable modules in a
`lit.local.cfg`.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D157625