This adds a function to `DiagnosticBuilder` to delay the entire diagnostic as a
bug to be emitted at a later time. This'll end up getting used in the compiler
in the subsequent commits...

This alters the return value of the `try_get` function so the error contains a
diagnostic rather than a `CycleError`. This way consumers are forced to take
*some* action (else they get a bug to an un-emitted diagnostic). This action
could be to emit the error itself, or in some cases delay the diagnostic as a
bug and continue.

The `sized_constraint` and `needs_drop_raw` queries both use `try_get` to detect
cycles, but in both of these cases the cycle indicates an error has happened
elsewhere in compilation. In these cases we can just delay the diagnostic to get
emitted as a bug later if we ended up forgetting to emit the error diagnostic.

It sounds like this is being handled elsewhere, so for now just preserve the
existing behavior of ignoring th error.

This seems like it may be likely to cause bugs with `RUST_LOG` and other
"interesting" scenarios, but it removes the usage of `try_get` for now!