[llvm] Add CMake configure preset building blocks (PR #170019)

[Richard Thomson via llvm-commits]

LegalizeAdulthood wrote:

> > Since I have commit access and this is purely opt-in for other developers and has no impact on existing builds or workflows, I'm going to merge this PR after it's been up for a week unless there are objections.
> 
> FWIW, I'm a bit uncomfortable with that reasoning -- when we have active maintainers for an area of the compiler being touched by a PR, we generally expect folks to wait for them to weigh in (thanks @petrhosek!) rather than assume that silence is acceptance, especially around the end of the year when lots of folks are out of the office. Of course, this is contextual (e.g., if something is on fire, fix it!) and I doubt there would have been problems had you landed without approval. More just something to keep in mind when changing stuff that has broader scope like this.

I agree with everything you said, was aware of that standing policy and already considered that before submitting this PR.  Since this PR has **no** functional change to anyone's workflow or any automated builds, must be opted-into in order to be used and only enhances developer workflows, I consider this falling into the exception rather than the rule.

https://github.com/llvm/llvm-project/pull/170019