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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> Sorry, I don't understand here, extern is a keyword, this `import` will be treated as `kw_import`

Exactly, in phase 4 we are supposed to transform the import into `kw_import`. In phase 7, we should get an error about an invalid module import declaration. I am getting no error at all with a build containing all of the commits from this PR except the latest one (which should make no difference on this point).

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