[Issue 21176] deprecation inside template should show the instance loc as errorSupplemental

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21176

--- Comment #2 from Basile-z <b2.temp at gmx.com> ---
`deprecated` is sometimes used to refactorize. In this case it's essential to
have the instantation sites as additional diagnostic.

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