pgsql: Fix performance bug from conflict between two previous improveme

Fix performance bug from conflict between two previous improvements.

My expanded-objects patch (commit 1dc5ebc9077ab742) included code to make
plpgsql pass expanded-object variables as R/W pointers to certain functions
that are trusted for modifying such variables in-place.  However, that
optimization got broken by commit 6c82d8d1fdb1f126, which arranged to share
a single ParamListInfo across most expressions evaluated by a plpgsql
function.  We don't want a R/W pointer to be passed to other functions
just because we decided one function was safe!  Fortunately, the breakage
was in the other direction, of never passing a R/W pointer at all, because
we'd always have pre-initialized the shared array slot with a R/O pointer.
So it was still functionally correct, but we were back to O(N^2)
performance for repeated use of "a := a || x".  To fix, force an unshared
param array to be used when the R/W param optimization is active.

Commit 6c82d8d1fdb1f126 is in HEAD only, so no need for a back-patch.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d3eaab3ef0d552a2f6555b0424a32dc9e77fb17c

Modified Files
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src/pl/plpgsql/src/pl_exec.c |   38 ++++++++++++++++++++++++++++++--------
1 file changed, 30 insertions(+), 8 deletions(-)