pgsql: Straighten out leakproofness markings on text comparison functio

Straighten out leakproofness markings on text comparison functions.

Since we introduced the idea of leakproof functions, texteq and textne
were marked leakproof but their sibling text comparison functions were
not.  This inconsistency seemed justified because texteq/textne just
relied on memcmp() and so could easily be seen to be leakproof, while
the other comparison functions are far more complex and indeed can
throw input-dependent errors.

However, that argument crashed and burned with the addition of
nondeterministic collations, because now texteq/textne may invoke
the exact same varstr_cmp() infrastructure as the rest.  It makes no
sense whatever to give them different leakproofness markings.

After a certain amount of angst we've concluded that it's all right
to consider varstr_cmp() to be leakproof, mostly because the other
choice would be disastrous for performance of many queries where
leakproofness matters.  The input-dependent errors should only be
reachable for corrupt input data, or so we hope anyway; certainly,
if they are reachable in practice, we've got problems with requirements
as basic as maintaining a btree index on a text column.

Hence, run around to all the SQL functions that derive from varstr_cmp()
and mark them leakproof.  This should result in a useful gain in
flexibility/performance for queries in which non-leakproofness degrades
the efficiency of the query plan.

Back-patch to v12 where nondeterministic collations were added.
While this isn't an essential bug fix given the determination
that varstr_cmp() is leakproof, we might as well apply it now that
we've been forced into a post-beta4 catversion bump.

Discussion: https://postgr.es/m/31481.1568303470@sss.pgh.pa.us

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d9110d7e14819c10049c9beb1034958787c77e79

Modified Files
--------------
src/backend/utils/adt/varlena.c          |   6 ++
src/include/catalog/catversion.h         |   2 +-
src/include/catalog/pg_proc.dat          | 132 +++++++++++++++----------------
src/test/regress/expected/opr_sanity.out |  54 ++++++++++---
src/test/regress/sql/opr_sanity.sql      |   2 -
5 files changed, 115 insertions(+), 81 deletions(-)