PS: Never hide string interpolate argument in path graphs

[MathiasVP]

This PR ensures that the argument of a string interpolation is always visible in dataflow paths. That is, previously a dataflow query going from source to sink in the following:

$x = source
$y = "Hello $x"
sink $y

would show the following steps:

1. source --> $x (on line 1)
2. $x (on line 1) --> $y (on line 2)
3. $y (on line 2) --> $y (on line 3)

With this change, we now see:

1. source --> $x (on line 1)
2. $x (on line 1) --> $x (on line 2)
3. $x (on line 2) --> "Hello $x"
4. "Hello $x" --> $y (on line 2)
5. $y (on line 2) --> $y (on line 3)

This makes it a lot easier to understand the paths in code that uses lots of string interpolation.