[libcxx] [llvm] Add libc++ github actions workflow to replace buildkite (PR #71836)

[via llvm-commits]

================
@@ -0,0 +1,180 @@
+# This file defines pre-commit CI for libc++ [bla bla bla].
----------------
EricWF wrote:

lol I didn't even catch that bit. My bad.

https://github.com/llvm/llvm-project/pull/71836