pgsql: Make pg_get_expr() more bulletproof.

Make pg_get_expr() more bulletproof.

Since this function is defined to accept pg_node_tree values, it could
get applied to any nodetree that can appear in a cataloged pg_node_tree
column. Some such cases can't be supported --- for example, its API
doesn't allow providing referents for more than one relation --- but
we should try to throw a user-facing error rather than an internal
error when encountering such a case.

In support of this, extend expression_tree_walker/mutator to be sure
they'll work on any such node tree (which basically means adding
support for relpartbound node types). That allows us to run pull_varnos
and check for the case of multiple relations before we start processing
the tree. The alternative of changing the low-level error thrown for an
out-of-range varno isn't appealing, because that could mask actual bugs
in other usages of ruleutils.

Per report from Justin Pryzby. This is basically cosmetic, so no
back-patch.

Discussion: https://postgr.es/m/20211219205422.GT17618@telsasoft.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6867f963e319934cbdafeb4bd5beaea5797c7be2

Modified Files
--------------
src/backend/nodes/nodeFuncs.c | 42 +++++++++++++++++++++++++++++++++++++
src/backend/optimizer/util/var.c | 11 ++++++++--
src/backend/utils/adt/ruleutils.c | 44 ++++++++++++++++++++++++++++++++++++++-
3 files changed, 94 insertions(+), 3 deletions(-)