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

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yronglin wrote:

@cor3ntin @ChuanqiXu9  Do you have any comments on the current implementation approach? Feel free to leave comments. Since this PR is too large, I'd like to split it into several smaller PRs if the implementation approach is correct. WDYT?

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