[llvm] [libcxx] [libc++][CI] Moves CI badge to main README. (PR #77247)

[Mark de Wever via llvm-commits]

mordante wrote:

> This makes sense to me -- I only wonder if we want to put this in a less prominent location since this is the README for the whole LLVM project, and the libc++ CI status is only relevant for a small portion of the project.

I checked and libc++ is currently the only scheduled CI job. I hope other subprojects will add their badges when they do so. I added @tstellar as reviewer since he reviewed the other badge we have.



https://github.com/llvm/llvm-project/pull/77247