pgsql: Fix pg_mcv_list_items() to produce text[]

Fix pg_mcv_list_items() to produce text[]

The function pg_mcv_list_items() returns values stored in MCV items. The
items may contain columns with different data types, so the function was
generating text array-like representation, but in an ad-hoc way without
properly escaping various characters etc.

Fixed by simply building a text[] array, which also makes it easier to
use from queries etc.

Requires changes to pg_proc entry, so bump catversion.

Backpatch to 12, where multi-column MCV lists were introduced.

Author: Tomas Vondra
Reviewed-by: Dean Rasheed
Discussion: https://postgr.es/m/20190618205920.qtlzcu73whfpfqne@development

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4d66285adc6bb4f9e4fd394d478d663cbccb5fc8

Modified Files
--------------
src/backend/statistics/mcv.c            | 128 ++++++++++++--------------------
src/include/catalog/catversion.h        |   2 +-
src/include/catalog/pg_proc.dat         |   2 +-
src/test/regress/expected/stats_ext.out |   6 +-
4 files changed, 51 insertions(+), 87 deletions(-)