Swift: Generate better declaration names

[MathiasVP]

Prior to this PR, the name we generate for the declarations:

func f(x first: Int, y second: Int, _ third: Int) { }
func f(z first: Int, w second: Int, _ third: Int) { }

were both just f. This makes it inconvenient to mark the first function overload as a taint source purely based on the name. In addition, the Swift compiler refers to each overload as f(x:y:_:) and f(z:w:_:) respectively.

With this PR, we generate the expected names (i.e., f(x:y:_:) and f(z:w:_:)) for the function overloads.

Note that as a special case of this, the name of a function f with no parameters:

func f() {  }

is actually f() with parentheses! That's a bit confusing to me since it looks like a function call (even though it's actually a reference to the function). This matches the output of swiftc, though.

This special case required a slight modification to swift/ql/test/library-tests/dataflow/dataflow/DataFlow.ql (which is included in c049988).

[AlexDenisov]

This looks so much better now! ❤️