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

[kadir çetinkaya via cfe-commits]

kadircet wrote:

> We can limit ourselves to only the paired source file, and cancel inclusions if the source file does not include a non-self-contained header directly. This will reduce the number of situations in which this hack works, but will avoid the problems you describe

deducing the paired source file is also a heuristic, which will likely go wrong at times. more importantly it'll still regress the behavior for self-contained headers, even if we ignore that, it still creates this new illusion of working when the user is lucky, and will trigger extra maintenance load (that's IMHO not justified) by already thin stretched maintainers.

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