accommodate for cloned bindparams w/ maintain_key

accommodate for cloned bindparams w/ maintain_key

Fixed issue where a bound parameter object that was "cloned" would cause a
name conflict in the compiler, if more than one clone of this parameter
were used at the same time in a single statement. This could occur in
particular with things like ORM single table inheritance queries that
indicated the same "discriminator" value multiple times in one query.

Fixes: #6824
Change-Id: Iba7a786fc5a2341ff7d07fc666d24ed790ad4fe8