Refactor constant evaluation to use a single error reporting function
that reports a type-error-like message.
Also, unify all error codes with the "constant evaluation error" message
to just E0080, and similarly for a few other duplicate codes. The old
situation was a total mess, and now that we have *something* we can
further iterate on the UX.
This turned up as part of #3170. When constructing an `undef` value to
return in the error case, we were trying to get the element type of the
Rust-level value being indexed instead of the underlying array; when
indexing a slice, that's not an array and the LLVM assertion failure
reflects this.
The regression test is a lightly altered copy of `const-array-oob.rs`.
