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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> The fact that it landed twice already could also be interpreted as having already flushed out most potential issues. And if it's still turns out to be a major problem, it's still possible to revert during the RC phase (but the opposite is not true, i.e. landing it during that time)

I don't think the feature is so important. It has been reverted twice doesn't mean it is stable now. It shouldn't matter to land it later.

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