inference: (slightly) improve type stability of capturing closures

[aviatesk]

As an idea to improve type stability for capturing closures, such as in #31909, I tried this idea of propagating the closure object as a PartialStruct whose fields include captured variables of which types are (partially) known. By performing const-prop on this closure::PartialStruct, we can achieve a certain level of type stability.
Specifically, I made some modifications to abstract_eval_new to allow creating PartialStruct even for :new objects that are !isconcretedispatch (since PartialStruct can now represent abstract elements). I also adjusted const_prop_argument_heuristic to perform aggressive constant propagation using such closure::PartialStruct.

As a result, the following code now achieves type stability:

julia> Base.infer_return_type((Bool,Int,)) do b, y
           x = b ? 1 : missing
           inner = y -> x + y
           return inner(y)
       end
Any                   # master
Union{Missing, Int64} # this commit

However, this alone was not enough to fully resolve #31909. The call graph of map is extremely complex, and simply applying constant propagation everywhere does not achieve the type safety requested in the issue.

Nevertheless this commit alone would still improve type stability for some cases, so I will go ahead and submit it as a PR.

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@nanosoldier runbenchmarks("inference", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.

[aviatesk]

@nanosoldier runbenchmarks("inference", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.