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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> > We don't need check `tok::string_literal`, just check `tok::header_name` is enouth.
> 
> I agree in principle. I think the standard has a defect: the wording makes it necessary to check `tok::string_literal` because `u8"string"` is a _string-literal_ but not a _header-name_. I will create an issue if I can't find one.

https://github.com/cplusplus/CWG/issues/769

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