[analyzer] introduce `RangeFactory.nodeWithPrecedingComments` extension

[asashour]

As hinted in https://dart-review.googlesource.com/c/sdk/+/277100/comment/e2373f61_88a6ed41/

It is better to have a utility method, used by both fixes.

[asashour]

https://dart-review.googlesource.com/c/sdk/+/278741

[srawlins]

cc @bwilkerson @keertip I think I really like this idea. What do you think?

[bwilkerson]

Having suggested it, I'm biased, but in favor, at least in general.

I'm still trying to decide whether I like the impact on the API (which is public, so we want to be careful to design something we want to support long term).

[bwilkerson]

Sorry. I looked again, and it isn't public API, so that isn't a concern.