[blocker fix]: use auto return in std.datetime

This is a blocker for https://github.com/D-Programming-Language/dmd/pull/4136

Without it, the error is:
```
std\datetime.d(17341): Error: function std.datetime.Interval!(TimeOfDay).Interval.end need 'this' to access member end
std\datetime.d(17374):        called from here: end()
std\datetime.d(18699): Error: template instance std.datetime.Interval!(TimeOfDay) error instantiating
std\datetime.d(17341): Error: function std.datetime.Interval!(DateTime).Interval.end need 'this' to access member end
std\datetime.d(17374):        called from here: end()
std\datetime.d(18700): Error: template instance std.datetime.Interval!(DateTime) error instantiating
std\datetime.d(18685): Error: pure nested function '__invariant63' cannot access mutable data '_begin'
std\datetime.d(18685): Error: need 'this' for '_begin' of type 'TimeOfDay'
std\datetime.d(18685): Error: pure nested function '__invariant63' cannot access mutable data '_end'
std\datetime.d(18685): Error: need 'this' for '_end' of type 'TimeOfDay'
```