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

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yronglin wrote:

> Can you please modify `clang/www/cxx_status.html` in your PR to indicate implementation of P1857 (and I assume P1703 which was subsumed)

Sure, updated!

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