[clang] Support C++20 Modules in clang-repl (PR #79261)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

@nikic, thanks for the details! 

Until this change we sailed fairly well with that change. It seems that the pcm rigorously records the clang notion of the triple which is probably what we want. However, if we are building a pcm in the context of `clang-repl` would it make sense to use the `getProcessTriple` notion for that pcm -- that'd be the behavior if we used header files instead of a module.

https://github.com/llvm/llvm-project/pull/79261