Fix incorrect strictness test for ArrayCoerceExpr expressions.

Fix incorrect strictness test for ArrayCoerceExpr expressions.

The recursion in contain_nonstrict_functions_walker() was done wrong,
causing the strictness check to be bypassed for a parse node that
is the immediate input of an ArrayCoerceExpr node.  This could allow,
for example, incorrect decisions about whether a strict SQL function
can be inlined.

I didn't add a regression test, because (a) the bug is so narrow
and (b) I couldn't think of a test case that wasn't dependent on a
large number of other behaviors, to the point where it would likely
soon rot to the point of not testing what it was intended to.

I broke this in commit c12d570fa, so back-patch to v11.

Discussion: https://postgr.es/m/27571.1550617881@sss.pgh.pa.us