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

[Mishal Shah via llvm-commits]

shahmishal wrote:

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

Compared the CI logs for this PR against the previous PR without runtime projects/tests enabled:

  - Without runtimes (previous PR): 1h 22m build+test, 94.45% sccache hit rate
  - With runtimes (this PR, compiler-rt): 1h 16m build+test, 99.89% sccache hit rate

Enabling runtimes projects/checks (compiler-rt) adds roughly 16 minutes of dedicated build+test time on top of the main project pipeline, isolated from caching effects.

```
  ┌─────────────────────────────────────────────────┬────────────────────┐
  │                      Phase                      │      Duration      │
  ├─────────────────────────────────────────────────┼────────────────────┤
  │ Configure + build compiler-rt                   │ ~2m 11s            │                                                                                                   
  ├─────────────────────────────────────────────────┼────────────────────┤
  │ Run check-compiler-rt test suite (4,482 tests)  │ 830.34s (~13m 50s) │
  ├─────────────────────────────────────────────────┼────────────────────┤
  │ Total added by LLVM_ENABLE_RUNTIMES=compiler-rt │ ~16 minutes        │
  └─────────────────────────────────────────────────┴────────────────────┘
```

> 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).

The queuing we noticed today is due to an issue we discovered with the infrastructure. Once we resolve the issue with the cloud provider, the situation should improve significantly. 

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