[NFC] [C++20] [Modules] Use new class CXX20ModulesGenerator to generate module...

[NFC] [C++20] [Modules] Use new class CXX20ModulesGenerator to generate module file for C++20 modules instead of PCHGenerator

Previously we're re-using PCHGenerator to generate the module file for
C++20 modules. But this is slighty more or less odd. This patch tries
to use a new class 'CXX20ModulesGenerator' to generate the module file
for C++20 modules.