[analyzer] Canonicalize SymIntExpr so the RHS is positive when possible

This PR changes the `SymIntExpr` so the expression that uses a
negative value as `RHS`, for example: `x +/- (-N)`, is modeled as
`x -/+ N` instead.

This avoids producing a very large `RHS` when the symbol is cased to
an unsigned number, and as consequence makes the value more robust in
presence of casts.

Note that this change is not applied if `N` is the lowest negative
value for which negation would not be representable.

Reviewed By: steakhal

Patch By: tomasz-kaminski-sonarsource!

Differential Revision: https://reviews.llvm.org/D124658