update world age between packages in e.g. using A, B

[JeffBezanson]

Fixes the issue reported here: https://discourse.julialang.org/t/combining-two-using-statements-is-2-5x-slower-than-separate-statements

[colintbowers]

It's possible that this will also fix #21653. I was able to trigger/fix #21653 by combining/uncombining using statements.

[ararslan]

Jeff says this likely doesn't apply to 0.6, since the issue does not appear possible on 0.6 due to differences in how this is handled between 0.6 and 0.7. @colintbowers, when you say you were able to trigger the issue, was that with 0.6 or 0.7?

[ararslan]

I'll remove the backport label for now, but if someone is certain that this fixes a known issue on 0.6 specifically then let me know.

[colintbowers]

@ararslan v0.6.2 (sorry should have included that), so sounds like the two are unrelated. Just thought I'd mention it given that both are world age issues that can be caused by combining/uncombining using. Thanks anyway.

[ararslan]

So I guess this is indeed an issue on 0.6 but perhaps this PR is not the solution that resolves it on 0.6.

[JeffBezanson]

That's really strange; in 0.6.x using A, B is lowered directly to two separate toplevel using statements.

[colintbowers]

Agreed that makes my "fix" all the weirder. On the plus side, I haven't encountered #21653 at all on v0.7.