[llvm] [CI] Run Linux premerge CI on libc++ changes. (PR #155188)

[Michael Buch via llvm-commits]

https://github.com/Michael137 requested changes to this pull request.

Hmmm libc++ folks had an issue with this back when we discussed running LLDB tests on libc++ CI. Mainly because of how long they might take to run. That's why we only run the data-formatter tests in the libc++ CI (https://github.com/llvm/llvm-project/pull/88312). But maybe that changed (or maybe I'm misunderstanding this current PR)

https://github.com/llvm/llvm-project/pull/155188