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

[Joe Loser via llvm-commits]

JoeLoser wrote:

@joker-eph sorry about approving/merging this early before engaging more with the community here on the feedback and approach.  As I talked with Jeff this morning, he reverted in https://github.com/llvm/llvm-project/commit/8846b91e15d4c8d280ee727c0f69b958f9b1440b and we'll re-evaluate how we want to proceed.  Sorry again, and we appreciate your feedback!

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