Replace Expr::getType() with ConstraintSystem::getType().

Update CSGen/CSApply/CSSolver to primarily use getType() from
ConstraintSystem.

Currently getType() just returns the type on the expression. As with
setType(), which continues to set the type on the expression, this
will be updated once all the other changes are in place.

This change also moves coerceToRValue from TypeChecker to
CosntraintSystem so that it can access the expression type map in the
constraint system.

Author: rudkx