pgsql: Rethink the "read/write parameter" mechanism in pl/pgsql.

Rethink the "read/write parameter" mechanism in pl/pgsql.

Performance issues with the preceding patch to re-implement array
element assignment within pl/pgsql led me to realize that the read/write
parameter mechanism is misdesigned.  Instead of requiring the assignment
source expression to be such that *all* its references to the target
variable could be passed as R/W, we really want to identify *one*
reference to the target variable to be passed as R/W, allowing any other
ones to be passed read/only as they would be by default.  As long as the
R/W reference is a direct argument to the top-level (hence last to be
executed) function in the expression, there is no harm in R/O references
being passed to other lower parts of the expression.  Nor is there any
use-case for more than one argument of the top-level function being R/W.

Hence, rewrite that logic to identify one single Param that references
the target variable, and make only that Param pass a read/write
reference, not any other Params referencing the target variable.

Discussion: https://postgr.es/m/4165684.1607707277@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1c1cbe279b3c6e8038c8f8079402f069bb4cea4c

Modified Files
--------------
src/backend/utils/adt/arrayfuncs.c |   7 +-
src/pl/plpgsql/src/pl_exec.c       | 177 ++++++++++++++++++-------------------
src/pl/plpgsql/src/pl_gram.y       |   6 +-
src/pl/plpgsql/src/plpgsql.h       |  12 ++-
4 files changed, 106 insertions(+), 96 deletions(-)