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

[Michael Spencer via cfe-commits]

Bigcheese 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?

The design was about what's required to be able to figure stuff out at the start of phase 4. `export module m; int n;` is easily handled by scanners.

If we really wanted we could fully duplicate the attribute grammar into the preprocessor to be able to fully restrict the line, but these weren't really intended to be preprocessor control lines. That was just a good way to restrict how they could be preprocessed.

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