pgsql: Implement type regcollation

Implement type regcollation

This will be helpful for a following commit and it's also just
generally useful, like the other reg* types.

Author: Julien Rouhaud
Reviewed-by: Thomas Munro and Michael Paquier
Discussion: https://postgr.es/m/CAEepm%3D0uEQCpfq_%2BLYFBdArCe4Ot98t1aR4eYiYTe%3DyavQygiQ%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a2b1faa0f279f18517adc1dd969e5f026eb51d33

Modified Files
--------------
doc/src/sgml/datatype.sgml            |  10 +++
doc/src/sgml/func.sgml                |  13 ++-
doc/src/sgml/ref/pgupgrade.sgml       |   1 +
src/backend/utils/adt/regproc.c       | 152 ++++++++++++++++++++++++++++++++++
src/bin/pg_upgrade/check.c            |   1 +
src/include/catalog/catversion.h      |   2 +-
src/include/catalog/pg_cast.dat       |  14 ++++
src/include/catalog/pg_proc.dat       |  15 ++++
src/include/catalog/pg_type.dat       |   4 +
src/test/regress/expected/regproc.out |  40 +++++++++
src/test/regress/sql/regproc.sql      |   7 ++
11 files changed, 256 insertions(+), 3 deletions(-)