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

[Nikita Popov via cfe-commits]

nikic wrote:

> Until this change we sailed fairly well. 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.

The pcm here is being generated by clang, not by clang-repl, so I'm not sure how we would know that we need to switch the triple.

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