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

[Louis Dionne via llvm-commits]

https://github.com/ldionne commented:

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.

But I totally agree that it doesn't belong in the documentation -- I think I had raised that concern in the original patch that added it and I appreciate that you took it on yourself to fix the issue!

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