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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Given we are going to branch clang19 in the end of the month, it is highly possible that we can't land sufficient patches before that. Note that even if we land this patch before that, the support of modules in clangd is expected to very slow for real world applications. See the summary of the patch for details. So we may miss the train again.

To mitigate this, I opened https://github.com/ChuanqiXu9/clangd-for-modules. This contains the following patches to make the clangd sufficient to work with modules in practical workloads. Feedbacks and contributions are welcomed. Hope this can be helpful for people who are waiting this.

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