Create a pg_shdepend entry for each role in TO clause of policies.

CreatePolicy() and AlterPolicy() omit to create a pg_shdepend entry for
each role in the TO clause. Fix this by creating a new shared dependency
type called SHARED_DEPENDENCY_POLICY and assigning it to each role.

Reported by Noah Misch. Patch by me, reviewed by Alvaro Herrera.
Back-patch to 9.5 where RLS was introduced.