perf(time_subtraction): avoid querying type/diag name of lhs twice

[ada4a]

changelog: none

[rustbot]

r? @y21

rustbot has assigned @y21.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Jarcho]

I have to question why this needs two different messages. Just "unchecked subtraction of a Duration" is enough to get the point across. Whether the lhs is a Duration or an Instant is immaterial to issue at hand (the subtraction can panic).