This lets us reduce noise by removing a bunch of `#[cfg]` attributes. In the
future, if we have tests for other front ends, we can create an `errors`
subdirectory, but for now, we only have the WGSL tests, so we should keep things
flat.

- added constructs
- added loop detection
- some logic from Tint borrowed for selection constructs

* Unify back::msl push constants map and sizes buffer

* Fix clippy warning

* Actually remove PushConstantsMap

* Update snapshots