pgsql: SQL/JSON: Fix some oversights in commit b6e1157e7

SQL/JSON: Fix some oversights in commit b6e1157e7

The decision in b6e1157e7 to ignore raw_expr when evaluating a
JsonValueExpr was incorrect.  While its value is not ultimately
used (since formatted_expr's value is), failing to initialize it
can lead to problems, for instance,  when the expression tree in
raw_expr contains Aggref nodes, which must be initialized to
ensure the parent Agg node works correctly.

Also, optimize eval_const_expressions_mutator()'s handling of
JsonValueExpr a bit.  Currently, when formatted_expr cannot be folded
into a constant, we end up processing it twice -- once directly in
eval_const_expressions_mutator() and again recursively via
ece_generic_processing().  This recursive processing is required to
handle raw_expr. To avoid the redundant processing of formatted_expr,
we now  process raw_expr directly in eval_const_expressions_mutator().

Finally, update the comment of JsonValueExpr to describe the roles of
raw_expr and formatted_expr more clearly.

Bug: #18657
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Diagnosed-by: Fabio R. Sluzala <fabio3rs@gmail.com>
Diagnosed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18657-1b90ccce2b16bdb8@postgresql.org
Backpatch-through: 16

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7148cb3e3067a21f98031ff4d01be8895b5d164e

Modified Files
--------------
src/backend/executor/execExpr.c       |  2 ++
src/backend/optimizer/util/clauses.c  | 24 ++++++++++++-----
src/include/nodes/primnodes.h         | 14 ++++++----
src/test/regress/expected/sqljson.out | 49 +++++++++++++++++++++++++++++++++++
src/test/regress/sql/sqljson.sql      | 14 ++++++++++
5 files changed, 92 insertions(+), 11 deletions(-)