[compiler] Collapse CompilerError.{invariant,simpleInvariant} (#35614)

`invariant()` was a pain to use - we always record a single location,
but the API required passing a compiler detail. This PR replaces
`invariant()` (keeping the name) with `simpleInvariant()`s signature,
and updates call sites accordingly. I've noticed that agents
consistently get invariant() wrong, which aligns with it being tedious
to call when you're writing code by hand. The simplified API should help
a bit.