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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

+1 to not landing until after the branch point; having more bake time (particularly for something that's been reverted a few times) is not a bad thing.

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