[naga] Remove non-essential override references via compaction

[andyleiserson]

Adds a mode to compaction that removes unused functions, global variables, and named types and overrides. This mode is used everywhere except the compaction at the end of lowering, where it is important to preserve unused items for type checking and other validation of the module.

Pruning all but the active entry point and then compacting makes process_overrides tolerant of missing values for overrides that are not used by the active entry point.

Fixes #5885

I have opened this as a self-contained pull-request for ease of reviewing, but in order to pass CI, it will need to be rebased on top of the changes in #7711.

Testing
Adds several cases in the validation test to verify that compaction removes references to non-essential overrides.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[andyleiserson]

There is a question what to do about the compact feature. Currently wgsl-in requires compact, so in many environments it is already required. But when wgsl-in is not set, we need a strategy for controlling the use of compaction in process_overrides:

1. Remove the compact feature and build the associated functionality unconditionally.
2. Make process_overrides conditional upon the compact feature, and arrange for the feature to be set when necessary. (Currently the no-features clippy runs in CI are broken, because wgpu-hal uses process_overrides, but does not enable compact.)
3. Make process_overrides available unconditionally, but when built without the compact feature, specifying all overrides is required.
4. Make process_overrides conditional upon a new feature (overrides?) which would depend on compact.

I have listed in my order of preference, but I don't have all the context about use cases and build size concerns that might justify one of my non-preferred options.

[andyleiserson]

It already came in useful to be able to pass the actual entry point here, and I have the changes to do that, but unless there's a desire to have them in this PR, I'll hold them for a follow-up so that this one doesn't get any bigger.