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

src/backend/optimizer/util/clauses.c

@@ -1179,9 +1179,8 @@ contain_nonstrict_functions_walker(Node *node, void *context)
 		 * the per-element expression is; so we should ignore elemexpr and
 		 * recurse only into the arg.
 		 */
-		return expression_tree_walker((Node *) ((ArrayCoerceExpr *) node)->arg,
-									  contain_nonstrict_functions_walker,
-									  context);
+		return contain_nonstrict_functions_walker((Node *) ((ArrayCoerceExpr *) node)->arg,
+												  context);
 	}
 	if (IsA(node, CaseExpr))
 		return true;