Split RelationClearRelation into three different functions

The old RelationClearRelation function did different things depending
on the arguments and circumstances. It could:

a) remove the relation completely from relcache (rebuild == false),
b) mark the entry as invalid (rebuild == true, but not in xact), or
c) rebuild the entry (rebuild == true).

Different callers used it for different purposes, and often assumed a
particular behavior, which was confusing. Split it into three
different functions, one for each of the above actions (one of them,
RelationInvalidateRelation, was already added in commit e6cd857).
Move the responsibility of choosing the action and calling the right
function to the callers.

Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://www.postgresql.org/message-id/9c9e8908-7b3e-4ce7-85a8-00c0e165a3d6%40iki.fi

Author: hlinnaka