Generate parameter name in sighelp when there is no public parameter name

[cartermp]

This fixes a quirk I noticed when dogfooding the feature in the F# compiler codebase.

In the compiler, we have several functions where the public signature doesn't give a parameter name, but the definition (obviously) has a parameter name. From a caller's standpoint, there is no argument to display, even though it still exists. This leads to tooltips that look like this:

The add2 function takes two parameters, but the public signature looks like this:

module internal Math

val add2: int -> int -> int

I would argue that this signature is the actual problem, but we still need to handle it in tools. The approach I went with is to just use a generic argX name:

Once the public signature is updated to have names, then those flow through in the tooltip instead.

[smoothdeveloper]

@cartermp, I don't know how related it is, but it would be great if

let f = fun a b -> a + b

would carry the parameter names to f.

[cartermp]

It's unrelated, but yeah that'd be cool

[KevinRansom]

@cartermp conflicts mate.