pgsql: Fix no-longer-valid shortcuts in expression_returns_set().

Fix no-longer-valid shortcuts in expression_returns_set().

expression_returns_set() used to short-circuit its recursion upon
seeing certain node types, such as DistinctExpr, that it knew the
executor did not support set-valued arguments for. That was never
inherent, though, just a reflection of laziness in execQual.c.
With the new implementation of SRFs there is no reason to think
that any scalar-valued expression node could not have a set-valued
subexpression, except for AggRefs and WindowFuncs where we know there
is a parser check rejecting it. And indeed, the shortcut causes
unexpected failures for cases such as a SRF underneath DistinctExpr,
because the planner stops looking for SRFs too soon.

Discussion: https://postgr.es/m/5259.1497044025@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8e72239e9d961c27f02b242e33fa832c364c7a4b

Modified Files
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src/backend/nodes/nodeFuncs.c | 30 +-----------------------------
src/test/regress/expected/tsrf.out | 8 ++++++++
src/test/regress/sql/tsrf.sql | 1 +
3 files changed, 10 insertions(+), 29 deletions(-)