[llvm] [CMake][LIT] Add option to run lit testsuites in parallel (PR #82899)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

Can you clarify why this is useful/desirable?

(it's good practice in general for the PR description to describe the context and motivation for the change, that is the "why" instead of "what" is done)

https://github.com/llvm/llvm-project/pull/82899