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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> But the pre-merge CI on the libc++ PR probably ran without being rebased on that change. So the test was skipped.

Github actions will run the changes as if they were merged into `main`, so the branch doesn't need to be rebased. It looks like the CI run was more stale than the landing of that PR though (the CI run was five days old, the PR landed four days ago), so it never picked it up.

If this is running as part of the bootstrapping build, that seems reasonable enough to me. I just want to make sure we aren't missing test coverage for these sorts of changes. This is the second or third time the lldb data formatters for libc++ have broken all of premerge for an extended length of time.

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