This improves the output for issue #72577, but there's still more work
to be done.
Currently, an overflow error during monomorphization results in an error
that points at the function we were unable to monomorphize. However, we
don't point at the call that caused the monomorphization to happen. In
the overflow occurs in a large recursive function, it may be difficult
to determine where the issue is.
This commit tracks and `Span` information during collection of
`MonoItem`s, which is used when emitting an overflow error. `MonoItem`
itself is unchanged, so this only affects
`src/librustc_mir/monomorphize/collector.rs`
