[NFC] Set C++20 Named Modules for CodeGen in ASTContext in the early place

Previously we'll set the named modules for ASTContext in ParseAST. But
this is not intuitive and we need comments to tell the intuition. This
patch moves the code the right the place, where the corrresponding
module is first created/loaded. Now it is more intuitive and we can use
the value in the earlier places.