[llvm] [Support] Use absolute paths for include filenames (PR #136687)

[Markus Böck via llvm-commits]

zero9178 wrote:

Not super familiar with the LSP spec unfortunately, but assuming there is no better place to convert to absolute paths it makes sense. 

Could this be tested in some way? E.g. that a URI now contains more components of a path that it previously didn't

https://github.com/llvm/llvm-project/pull/136687