[clang-tools-extra] [clangd] [C++20] [Modules] Introduce persistent cache for clangd built module file (PR #193883)

[Michael Spencer via cfe-commits]

Bigcheese wrote:

It would be good to have some discussion about this approach. Caching PCMs is very difficult to get right, and it would be nice to avoid having lots of different ways of doing it. I haven't taken a deep look at this patch yet, but it looks like it's taking the same approach as implicitly built modules.

https://github.com/llvm/llvm-project/pull/193883