[libcxx] [llvm] [libc++] Remove official Clang 18 support. (PR #130142)

[Mark de Wever via llvm-commits]

mordante wrote:

> Sorry for the delay, our workers are using 19.1.7 now and should pass the next build you do.
> 
> As of 2 weeks ago there were no problems with the test suite on Arm with this clang, but I am available to debug anything you might find now.

Thanks a lot for updating the bots!

https://github.com/llvm/llvm-project/pull/130142