Several more tweaks to signature help

[cartermp]

Addresses the following additional cases in #10941

type C() =
    member _.M(x) = ()


let c = C()
c.M(sqrt)// <<- caret is after 't'!

Before this PR: tooltip for M
After this PR: tooltip for sqrt

    /// Tries to cast an object to a given type; throws with the given error message if it fails.
    let tryConvert<'T> (descriptor: string) (value: obj) : 'T =
        try
            Convert.ChangeType(value, typeof<'T>) :?> 'T
        with ex ->
            let msg = sprintf "Unable to convert '%s' with value %A" descriptor value
            raise (new Exception(msg, ex))

    // Tooltip does NOT show when the generic argument is present:
    let result= (box 123) |> tryConvert<string> // <<- caret is here

Before this PR: no tooltip
After this PR: tooltip for tryConvert

[cartermp]

This is actually the most impactful change. It's what enables triggering function signature help when in a parenthesized context without any quirks.