C++: Fix interface for GuardCondition.comparesEq and GuardCondition.ensuresEq

[MathiasVP]

I got the interface slightly wrong in #15958: In

codeql/cpp/ql/lib/semmle/code/cpp/controlflow/IRGuards.qll

Line 143 in 219cd4e

predicate comparesEq(Expr e, int k, boolean areEqual, boolean testIsTrue) { none() }

it doesn't make sense to simply have a boolean to determine whether "the test was true or false" since the "test" may be the scrutinee of a switch. So instead, this PR updates the newly added predicates to instead have an AbstractValue column. The effect of this is visible in the second commit where we previously had:

| 61 | i == 0 when i is true |
| 61 | i == 1 when i is true |
| 61 | i == 2 when i is true |

which made no sense.

Note that this isn't a breaking change since the new predicates has only been there for about ... 30 minutes 😂

[MathiasVP]

DCA looks uneventful 🎉