pgsql: Fix crasher bug in array_position(s)

Fix crasher bug in array_position(s)

array_position and its cousin array_positions were caching the element
type equality function's FmgrInfo without being careful enough to put it
in a long-lived context. This is obviously broken but it didn't matter
in most cases; only when using arrays of records (involving record_eq)
it becomes a problem. The fix is to ensure that the type's equality
function's FmgrInfo is cached in the array_position's flinfo->fn_mcxt
rather than the current memory context.

Apart from record types, the only other case that seems complex enough
to possibly cause the same problem are range types. I didn't find a way
to reproduce the problem with those, so I only include the test case
submitted with the bug report as regression test.

Bug report and patch: Junseok Yang
Discussion: https://postgr.es/m/CAE+byMupUURYiZ6bKYgMZb9pgV1CYAijJGqWj-90W=nS7uEOeA@mail.gmail.com
Backpatch to 9.5, where array_position appeared.

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/79c89f1f4e939b7a3f3bb4a76476dcda651cd58b

Modified Files
--------------
src/backend/utils/adt/array_userfuncs.c | 6 ++++--
src/test/regress/expected/arrays.out | 14 ++++++++++++++
src/test/regress/sql/arrays.sql | 9 +++++++++
3 files changed, 27 insertions(+), 2 deletions(-)