[clang-tools-extra] [clangd] Add includes from source to non-self-contained headers (PR #72479)

[via cfe-commits]

sr-tream wrote:

> @sr-tream just so you know, every time you force-push a PR, github sends every subscriber an email notification

Sorry for flood, I will try to avoid this from now on

https://github.com/llvm/llvm-project/pull/72479