pgsql: Fix constant-folding of ROW(...) IS [NOT] NULL with composite fi

Fix constant-folding of ROW(...) IS [NOT] NULL with composite fields.

The SQL standard appears to specify that IS [NOT] NULL's tests of field
nullness are non-recursive, ie, we shouldn't consider that a composite
field with value ROW(NULL,NULL) is null for this purpose.
ExecEvalNullTest got this right, but eval_const_expressions did not,
leading to weird inconsistencies depending on whether the expression
was such that the planner could apply constant folding.

Also, adjust the docs to mention that IS [NOT] DISTINCT FROM NULL can be
used as a substitute test if a simple null check is wanted for a rowtype
argument.  That motivated reordering things so that IS [NOT] DISTINCT FROM
is described before IS [NOT] NULL.  In HEAD, I went a bit further and added
a table showing all the comparison-related predicates.

Per bug #14235.  Back-patch to all supported branches, since it's certainly
undesirable that constant-folding should change the semantics.

Report and patch by Andrew Gierth; assorted wordsmithing and revised
regression test cases by me.

Report: <20160708024746.1410.57282@wrigleys.postgresql.org>

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d2ef7758d2d2175509b2f49f7049e06ccd81fd57

Modified Files
--------------
doc/src/sgml/func.sgml                 | 70 +++++++++++++++++-----------------
src/backend/executor/execQual.c        | 15 ++++++++
src/backend/optimizer/util/clauses.c   | 13 +++++--
src/test/regress/expected/rowtypes.out | 54 ++++++++++++++++++++++++++
src/test/regress/sql/rowtypes.sql      | 24 ++++++++++++
5 files changed, 136 insertions(+), 40 deletions(-)