constprop: Add facility for widening arguments before constprop

[Keno]

There are various situations where we may want to constprop with something other than the most precise concrete arguments in order to make the resulting cache more useful to other call sites. One particular example of this might be a method where non-concrete inference already discovered that one argument is unused:

function foo(a, b::DispatchOnly)
    expensive_to_compile(a)
end

Right now, we will generally perform constprop for every different value of b, even though we already have the information that b is unused.

Another example is external absints that may want to treat certain types fully symbolically. They may want to substitute concrete values for an abstract domain.

This adds the facility to do both of these things by

1. Adding an appropriate interp hook in the constprop path
2. Adding a WidendedSimpleArgtypes wrapper that's like SimpleArgtypes but works around an issue where we would override cache information using values from concrete eval, which is not legal if the argtypes were widened.

[aviatesk]

It could be a valuable move to use WidenedArgtypes for boosting cache hits when there are unused arguments. Should I go ahead and try implementing it (maybe it not be that simple as it likely requires something like EA though)?

[Keno]

It could be a valuable move to use WidenedArgtypes for boosting cache hits when there are unused arguments. Should I go ahead and try implementing it (maybe it not be that simple as it likely requires something like EA though)?

Yes, that's the natural extension of this PR - I just didn't try it yet. I think EA may compute it as it goes, but yeah, it's not quite the same analysis.