A macro for triggering an ICE.
Calling bug instead of panicking will result in a nicer error message and should
therefore be preferred over panic/unreachable or others.
A macro for triggering an ICE with a span.
Calling span_bug! instead of panicking will result in a nicer error message and point
at the code the compiler was compiling when it ICEd. This is the preferred way to trigger
ICEs.