Don't create a type parameter in the closure for captured @nospecialize arguments

[xal-0]

When we capture a variable without boxing, we always generate a type parameter for the closure. This probably isn't what the user wants if the captured variable is an argument marked @nospecialize.

Before:

julia> K(@nospecialize(x)) = @nospecialize(y) -> x
K (generic function with 1 method)

julia> f, g = K(1), K("a")
(var"#K##0#K##1"{Int64}(1), var"#K##0#K##1"{String}("a"))

julia> f(2), g(2)
(1, "a")

julia> methods(f)[1].specializations
svec(MethodInstance for (::var"#K##0#K##1"{Int64})(::Any), MethodInstance for (::var"#K##0#K##1"{String})(::Any), nothing, nothing, nothing, nothing, nothing)

julia> fieldtypes(typeof(f)), fieldtypes(typeof(g))
((Int64,), (String,))

After:

julia> K(@nospecialize(x)) = @nospecialize(y) -> x
K (generic function with 1 method)

julia> f, g = K(1), K("a")
(var"#K##0#K##1"(1), var"#K##0#K##1"("a"))

julia> f(2), g(2)
(1, "a")

julia> methods(f)[1].specializations
MethodInstance for (::var"#K##0#K##1")(::Any)

julia> fieldtypes(typeof(f)), fieldtypes(typeof(g))
((Any,), (Any,))

[topolarity]

Broadly looks good to me (although I really don't know lowering)

I think this is much more expected behavior. Thanks for the tune-up!