Uses of `ConstantValueStore::GetConstantInstId` or
`ConstantValueStore::GetInstId` in eval indicate that the code expects a
constant value. Instead of just ending up with `None` in strange places,
diagnose this and convert to an `ErrorInst` when expectations are not
met.
We add `RequireConstantValue` to pair with `GetConstantValue`, and
rename `GetConstantValueIgnoringPeriodSelf` to
`RequireConstantValueIgnoringPeriodSelf` since the former would just be
unused.
Adds a test where a runtime value ends up in the RHS of a rewrite
constraint, where a constant value is expected. This issue was uncovered
by a fuzzer.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
