self-referential c record imports

examples/interop/recursive/app/main.bro

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+native :: import "../include/native.h"
+
+# Exercises a self-referential imported C record (`struct Node { struct Node *next; int value; }`):
+# the importer must not recurse forever populating it, and the checker must accept the
+# self-referential `?*mut Node` field as C-layout-compatible.
+
+main :: func() i32 {
+	node native.Node = native.Node { next = none, value = 7 }
+	if node.next |_| {
+		return 1
+	}
+	if node.value == 7 {
+		return 0
+	}
+	return 2
+}