[llvm] [LLVM] Remove the requirement for named pipe in jobserver (PR #169154)

[Michał Górny via llvm-commits]

mgorny wrote:

Rebased and added a test so far. I've just copied the whole logic from `UnixClientFifo` test, since it also works fine for an empty file.

https://github.com/llvm/llvm-project/pull/169154