[Issue 13517] Only touch .di generated file if changed

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13517

--- Comment #5 from Andrei Alexandrescu <and...@erdani.com> ---
Again: hoisting this into the compiler lowers the barrier of entry and
simplifies the life of anyone building D code with .di files.

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