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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

It doesn't look like it. https://github.com/llvm/llvm-project/pull/94379 landed with all CI passing despite there being an LLDB failure that the premerge bots then caught. https://lab.llvm.org/staging/#/builders/192/builds/4005

We can do it either way, but I think having the coverage here is important, ideally to coordinate patches between LLDB and libc++.

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