[clang] [clang][Tooling] Support relative directory in compilation database (PR #127734)

[kadir çetinkaya via cfe-commits]

kadircet wrote:

> In my case I am using my build system (bazel) to generate the compile_commands.json file as part of the build. At this point given bazel's sandboxing / hermiticity model, the user's checkout PWD is unknown. This file is then copied somewhere and used by clangd. Today to make that work it requires post processing the directory field, replacing some sentinel with the real value.

thanks for the details! I don't think that value can be arbitrary though. bazel still runs its builds from a certain directory (called [execution_root](https://bazel.build/docs/user-manual#:~:text=below%20this%20directory.-,execution_root,-%3A%20the%20absolute%20path)) and the rest of the compile flags you have (unless you're also modifying the flags) are only guaranteed to work when combined with this `execution_root`. as any relative paths in the compile flags are relative to this path.

> I was actually surprised by the limitation of only supporting absolute paths today given that from the user perspective I provided the tools a totally valid path, where my source files relative to ., VSCode launches clangd from the root of the repo, and then the paths didn't work.

I guess such convenience features doesn't really matter for json compilation databases most of the time, as it's usually generated by build systems and not humans.

https://github.com/llvm/llvm-project/pull/127734