Rollup merge of #153033 - Zalathar:ensure, r=nnethercote

Clarify how "ensure" queries check whether they can skip execution

The current `check_cache: bool` field in QueryMode is confusing for a few reasons:

- It actually refers to checking the *disk cache*, not the in-memory cache.
- It gives no indication of what condition is being checked, or why.
- It obscures the link between `tcx.ensure_ok()` and `tcx.ensure_done()`, and the actual check.

This PR replaces that field with an `EnsureMode` enum that distinguishes between ensure-ok and ensure-done, and leaves the actual check as an implementation detail of the ensure-done mode.

This PR also renames `ensure_must_run` → `check_if_ensure_can_skip_execution`, and uses a return struct to more clearly indicate how this helper function gives permission for its caller to skip execution of a query that would otherwise be executed. This also inverts the sense of the returned boolean, which was previously “must run” but is now ”skip execution”.

r? nnethercote (or compiler)

Author: JonathanBrouwer