pgsql: Improve handling of array elements as getdiag_targets and cursor

Improve handling of array elements as getdiag_targets and cursor_variables.

There's no good reason why plpgsql's GET DIAGNOSTICS statement can't
support an array element as target variable, since the execution code
already uses the generic exec_assign_value() function to assign to it.
Hence, refactor the grammar to allow that, by making getdiag_target
depend on the assign_var production.

Ideally we'd also let a cursor_variable expand to an element of a
refcursor[] array, but that's substantially harder since those statements
also have to handle bound-cursor-variable cases. For now, just make sure
the reported error is sensible, ie "cursor variable must be a simple
variable" not "variable must be of type cursor or refcursor". The latter
was quite confusing from the user's viewpoint, since what he wrote
satisfies the claimed restriction.

Per bug #14463 from Zhou Digoal. Given the lack of previous complaints,
I see no need for a back-patch.

Discussion: https://postgr.es/m/20161213152548.14897.81245@wrigleys.postgresql.org

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/55caaaeba877eac1feb6481fb413fa04ae9046ac

Modified Files
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src/pl/plpgsql/src/pl_gram.y | 35 +++++++++++++++++++++--------------
src/test/regress/expected/plpgsql.out | 10 ++++++----
src/test/regress/sql/plpgsql.sql | 10 ++++++----
3 files changed, 33 insertions(+), 22 deletions(-)