PATCH: Support nested visibility push

[Mark Mitchell]

H. J. Lu wrote:

> 
> I can change the wording of the limit. BTW, I don't think VEC supports
> pushing 400 same value very efficiently. But I can change it to use
> VEC if it is what takes to support 400 nested push of the same
> visibility.

Change it to use VEC.  Dan has given good reasons and I agree.  And it's 
general GNU policy not to have hard limits on things; let's not do so here.

-- 
Mark Mitchell
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