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

[Michael Spencer via cfe-commits]

Bigcheese wrote:

> This is ill-formed because we trait identifier `import` as kw_import. @Bigcheese
> 
> ```c++
> typedef int import;
> extern
> import and x;
> ```

Hmm, because `and` is not a "header-name, `<`, identifier, string-literal, or `:` preprocessing token" this should not be treated as a kw_import. I assume it's being treated as an identifier even though it's an alternative token.

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