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

[Jeff Niu via llvm-commits]

Mogball wrote:

@patrickdoc @JoeLoser Can we try what Mehdi is suggesting instead of what is enabled by this PR? If that works, I don't see a need for this PR and we can revert it, since there is no clear use case.

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