Add source position information to target of CtFieldAccess

In case of CtFieldAccess, the position of target expression are inherited from its parents. For example, when an expression "var.field" is compiled, var(it's a CtFieldAccess or CtVariableAccess) is set as target of var.field(it's a CtFieldAccess). But position of var is not set. This commit set the position of var as its real position instead of that of its parent.
Don't know if it's a feature that you need or expected, however. :)