Create different data for each Literal when creating fake data.

Create different data for each Literal when creating fake data.
Thread a generator through the functions for creating fake arguments so the same
generator can be reused which avoids repeating the same data patterns for each
argument generated.

Also tweak the position-dependent biasing heuristic to create both positive and
negative numbers for small literals.

PiperOrigin-RevId: 183473588