[clang] [C++20][Modules] Implement P1857R3 Modules Dependency Discovery (PR #107168)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

> If repeating 9 RUN lines three times is what needs to be done, I can live with that. I'm just a bit worried for maintainability of that test, because 9 RUN lines per language mode is much higher than you usual DR test. `-verify-strict` is strict enough that I'd merge what can be merged today, and if in the future we cut the lexing off more aggressively, the test will fail, so there's no compromise on the integrity of the test.

Yup, i don't think there is much of a choice here!

https://github.com/llvm/llvm-project/pull/107168