Don't create a type parameter in the closure for captured @nospecialize arguments (#58426)
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,))
```
julia
52e14ccc - Don't create a type parameter in the closure for captured @nospecialize arguments (#58426)
