[clang-tools-extra] [clangd] [C++20] [Modules] Introduce initial support for C++20 Modules (PR #66462)

[Chuanqi Xu via cfe-commits]

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ChuanqiXu9 wrote:

Oh, I don't know why I didn't get this in files page so I missed this.

But since we can't get rid of writing/reading the modules actually in `ModulesBuilder` (Or it is pretty hard). Then it looks not so worthy to introduce the layer. 

https://github.com/llvm/llvm-project/pull/66462