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

[via Digitalmars-d-bugs]

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

--- Comment #18 from Steven Schveighoffer <schveiguy at yahoo.com> ---
I would add a stipulation that if the purpose of the compiler invocation is
ONLY to build the .di file, then the compiler should rewrite it. So dmd -c -o-
-H should still touch the .di file even if it's identical to the existing one.

Otherwise, this disfavors a build system where the .di is built as a separate
step (and causes a continual "rebuild" of a .di file every time the build is
called).

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