[llvm] [lit] Add --per-process-output-dir to isolate test_exec_root across concurrent runs (PR #206826)

[James Henderson via llvm-commits]

jh7370 wrote:

I think more generally useful to me would be to add an option to specify the test output directory for an invocation of lit. This means it's not tied to the process ID specifically and people can use it for other reasons (e.g. CI job IDs etc). Another advantage with that approach is that it could allow reusing of the build directory between runs, meaning you don't completely lose the benefit of things like test run ordering based on previous test runs, because you could reuse the test directories between runs of the script.

https://github.com/llvm/llvm-project/pull/206826