[clang-tools-extra] [clangd] [C++20] [Modules] Add scanning cache (PR #125988)

[kadir çetinkaya via cfe-commits]

https://github.com/kadircet requested changes to this pull request.

thanks, I think the biggest concern is about initial performance of this approach until cache warms up.

also can we have some tests? it should be possible to unittest by injecting a custom CDB into modules builder.

https://github.com/llvm/llvm-project/pull/125988