pgsql: Add read support for some missing raw parse nodes

Add read support for some missing raw parse nodes

The node types A_Const, Constraint, and A_Expr had custom output
functions, but no read functions were implemented so far.

The A_Expr output format had to be tweaked a bit to make it easier to
parse.

Be a bit more cautious about applying strncmp to unterminated strings.

Also error out if an unrecognized enum value is found in each case,
instead of just printing a placeholder value.  That was maybe ok for
debugging but won't work if we want to have robust round-tripping.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/4159834.1657405226@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a6bc3301925e1a8ad1f58da629b9dd55bc4b8d9c

Modified Files
--------------
src/backend/nodes/outfuncs.c   |   9 +-
src/backend/nodes/readfuncs.c  | 249 ++++++++++++++++++++++++++++++++++++++++-
src/include/nodes/parsenodes.h |   6 +-
3 files changed, 253 insertions(+), 11 deletions(-)