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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> It is hard to believe it is by design to allow "export module m; int n;" while we reject others. Is is possible to adjust the wording for it?

Why is that hard to believe? The feature originally was not envisioned as a preprocessor directive, so the "line" aspect is not strongly enforced. The later adjustments made "minimal" changes, which can be seen as a sign of intent not to affect the validity of existing code if said code was not "problematic" within the scope of the problems that the papers/issues were trying to address.

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