[clang][dataflow] Store flow condition constraints in a single `FlowConditionConstraints` map.

A flow condition is represented with an atomic boolean token, and it is bound to a set of constraints: `(FC <=> C1 ^ C2 ^ ...)`. \
This was internally represented as `(FC v !C1 v !C2 v ...) ^ (C1 v !FC) ^ (C2 v !FC) ^ ...` and tracked by 2 maps:
- `FlowConditionFirstConjunct` stores the first conjunct `(FC v !C1 v !C2 v ...)`
- `FlowConditionRemainingConjuncts` stores the remaining conjuncts `(C1 v !FC) ^ (C2 v !FC) ^ ...`

This patch simplifies the tracking of the constraints by using a single `FlowConditionConstraints` map which stores `(C1 ^ C2 ^ ...)`, eliminating the use of two maps.

Reviewed By: gribozavr2, sgatev, xazax.hun

Differential Revision: https://reviews.llvm.org/D128357