Callsite @constprop support

[aviatesk]

Like the statement @[no]inline support, we can add support for callsite @constprop annotation. That should allow us to something like:

julia> @noinline f_constprop_simple(f, x) = (f(x); Val{x}())
f_constprop_simple (generic function with 1 method)

julia> @test Base.return_types((Function,)) do f
           Base.@constprop :aggressive f_constprop_simple(f, 1)
       end |> only == Val{1}