Simple constant folding for bounds upper offsets

[kkjeer]

This PR introduces a temporary solution for an issue with bounds validation: the bounds checker should be able to prove that bounds(p, p + ((len + 1) - 1)) imply bounds(p, p + len).

The solution involves performing constant folding for a limited set of expressions. If the upper offset of a range is of the form (e +/- a) +/ b, where a and b are integer constants and e is an expression, we extract the variable part as e and the constant part as a + b (or a + -b, -a + b, or -a + -b).

This is intended to be a temporary fix until we can solve issues with constant folding, commutativity, and associativity in bounds expressions in a more general way.

This fix addresses +1/-1 situations that can arise from passing arguments to functions with declared parameter bounds (see checkedc/450). However, it does not fix +1/-1 situations that can occur as a result of bounds widening, e.g.

void f(_Nt_array_ptr<char> p : count(len), unsigned int len) {
  if (*(p + len)) {
    ++len;
  }
}

The bounds checker is unable to create a base range for the inferred bounds bounds(p, (p + (len - 1)) + 1). We need further work to address associativity issues with bounds expressions.

[mgrang]

Nit: Extra semicolon.

[sulekhark]

LGTM! Thank you!

[mgrang]

LGTM.