[Issue 21593] Only update file time if file to be written already exists

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21593

--- Comment #4 from Andrei Alexandrescu <andrei at erdani.com> ---
Related: https://issues.dlang.org/show_bug.cgi?id=21596

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