pgsql: Need to do SPI_push/SPI_pop around expression evaluation in plpg

Need to do SPI_push/SPI_pop around expression evaluation in plpgsql.

We must do this in case the expression evaluation results in calling
another plpgsql function (or, really, anything using SPI).  I missed
the need for this when I converted exec_cast_value() from doing a
simple InputFunctionCall() to doing ExecEvalExpr() in commit 1345cc67b.
There is a SPI_push_conditional in InputFunctionCall(), so that there
was no bug before that.

Per bug #14414 from Marcos Castedo.  Add a regression test based on his
example, which was that a plpgsql function in a domain check constraint
didn't work when assigning to a domain-type variable within plpgsql.

Report: <20161106010947.1387.66380@wrigleys.postgresql.org>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/674877e93a1b9217c692a5671e1118136959ee74

Modified Files
--------------
src/pl/plpgsql/src/pl_exec.c          |  4 ++++
src/test/regress/expected/plpgsql.out | 19 +++++++++++++++++++
src/test/regress/sql/plpgsql.sql      | 22 ++++++++++++++++++++++
3 files changed, 45 insertions(+)