[clang] Reapply "[C++20][Modules] Implement P1857R3 Modules Dependency Discovery" (#173130)" (PR #173789)

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yronglin wrote:

> [P1857R3 Modules Dependency Discovery](https://wg21.link/p1857r3).

@nikic I have created a new PR to fix this regression https://github.com/llvm/llvm-project/pull/177153. Could you help verify that this PR can reduce this compile time? Many thanks!

https://github.com/llvm/llvm-project/pull/173789