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

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yronglin wrote:

> > friendly ping~
> 
> Unfortunately, I was not able to get to this before starting end-of-year holidays. I'm not entirely sure that I'll be able to get back to this before January.

Thanks! Got it.

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