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

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yronglin wrote:

> @yronglin Please ping me when you need reviews. Thanks!

Thanks, I think it's ready for review now.

The latest commit remove `tok::annot_module_name` token kind, because we want to do error recovery and continue parsing when the module/import directive is ill-formed after P1857R3.

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