Except as provided in subsection (b) of this section, while a trust is revocable, rights of the beneficiaries are subject to the control of the settlor and the duties of the trustee are owed exclusively to the settlor. While a trust is revocable and a settlor does not have the capacity to revoke the trust, a beneficiary to which distributions may be made during the lifetime of the settlor shall have the right to enforce the trust as if the trust were irrevocable.