r225570 - Don't emit implicit template instantiations eagerly (PR21718)

[Rafael Espíndola]

>> A nice step towards fixing pr16187 :-)
>
> I'm not sure I fully understand the details of that bug. Are you
> thinking we could solve it if we delay more kinds of declarations so
> the visibility gets computed at the end?
>

Correct. To fix it we would have to delay the computation of any
visibility based on a decl that can go from implicitly hidden
(-fvisibility=hidden) to explicitly default (attribute).

With that we could then cache the visibility computation, which would
hopefully pay for the cost of delaying the computation.

Cheers,
Rafael