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

[ada4a]

changelog: none

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[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).

[ada4a]

Fair point -- opened a v2 at #16097