[Issue 18906] Template specialisations should not be stripped if they're not called

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18906

--- Comment #2 from Manu <turkeyman at gmail.com> ---
It's a specialisation.

Someone might call `t!(IDontKnowWhat)(value);`, and if IDontKnowWhat is int, it
should use that specialisation, and if it's not, it should instantiate the
generic unspecialised one.

It also affects function mangling, and extern linkage.

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