[llvm] [premerge] Build and check runtimes on macOS premerge (PR #210852)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Do you have numbers on how much this increases the job run time for patches that touch `.ci` (which should run everything)?

Touching LLVM runs a job that takes ~an hour which is already pretty long, and there was quite a bit of queuing today. There are no official guidelines for how long premerge should take to run (I really need to submit the RFC I have written up), but for Linux/Windows x86 we were targeting <30 minutes for queue+run (although we're a bit behind that currently).

https://github.com/llvm/llvm-project/pull/210852