Upgrade PostgreSQL-12 to rc1

There have been many bug fixes for PostgreSQL 12 reported during the Beta 4
period and applied to this release candidate. These include:

Add additional "leakproof" markings to certain string functions to better
support nondeterministic collations. This can positively impact the performance
of some query plans

Removal of the ECPG DECLARE STATEMENT functionality

The ecpglib major version change was reverted

Fix handling of nondeterministic collations with pattern_ops opclasses

Release notes: https://www.postgresql.org/about/news/1975/