[PBQP] Replace PBQPBuilder with composable constraints (PBQPRAConstraint).

This patch removes the PBQPBuilder class and its subclasses and replaces them
with a composable constraints class: PBQPRAConstraint. This allows constraints
that are only required for optimisation (e.g. coalescing, soft pairing) to be
mixed and matched.

This patch also introduces support for target writers to supply custom
constraints for their targets by overriding a TargetSubtargetInfo method:

std::unique_ptr<PBQPRAConstraints> getCustomPBQPConstraints() const;

This patch should have no effect on allocations.