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

[via cfe-commits]

sr-tream wrote:

> apart from technical details like the threading concerns and reliance on certain variants that we don't really have (eg order of includes); at a high level the idea of "finding a representative source file for the header and replicating the PP state" is hard to work in general.
> 
> The current approach of finding a candidate through `HeaderIncluders` will pick an arbitrary source file that transitively includes the header you're interested in. Hence you'll actually create a set of `-include XX` commands, that **might** (and probably will) recursively include the header itself. Also there's nothing detecting a failure in processing of the compilation unit, this is done without checking self-containedness (which is again hard to check) hence will definitely regress both performance and correctness for self-contained headers as well.

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

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