Race condition when using go-cmp on structs that include `time.Time`

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This a follow-up to https://github.com/golang/go/issues/74460 in the golang issues tracker, where go present a data race when some of the private fields of time.Time are accessed trough the `reflect` package.

The Golang team decided not to fix the issue as it have implication on the maintainability of private fields in the standard library and suggested to fix `go-cmp` to treat `time.Time` as a special case and use `==` to compare time fields as that's the way go recommend comparing `time.Time` (read comments on https://github.com/golang/go/issues/74460 for more details).