Rollup merge of #151976 - Zalathar:collect-active-jobs, r=nnethercote

Rename `collect_active_jobs` to several distinct names

Key renames:
- Function `collect_active_jobs` → `collect_active_jobs_from_all_queries` (method in trait `QueryContext`)
- Constant `COLLECT_ACTIVE_JOBS` → `PER_QUERY_GATHER_ACTIVE_JOBS_FNS` (list of per-query function pointers)
- Function `collect_active_jobs` → `gather_active_jobs` (per-query function in `query_impl::$name`)
- Function `collect_active_jobs` → `gather_active_jobs_inner` (method in `QueryState`)
  - Giving these four things distinct names makes it a lot easier to tell them apart!
  - The switch from “collect” (all queries) to “gather” (single query) is intended to make the different parts a bit more memorable and searchable; I couldn't think of a more natural way to express this distinction so I settled for two different synonyms.

There should be no change to compiler behaviour.

r? nnethercote (or compiler)

Author: Zalathar