expand out Index if passed to "constraint"

expand out Index if passed to "constraint"

Fixed bug where the PostgreSQL
:paramref:`_postgresql.OnConflictClause.constraint` parameter would accept
an :class:`.Index` object, however would not expand this index out into its
individual index expressions, instead rendering its name in an ON CONFLICT
ON CONSTRAINT clause, which is not accepted by PostgreSQL; the "constraint
name" form only accepts unique or exclude constraint names. The parameter
continues to accept the index but now expands it out into its component
expressions for the render.

Fixes: #9023
Change-Id: I6baf243e26bfe578bf3f193c162dd7a623b6ede9