Add a regression test that exports can expose unexported items

While those unexported items can't be used by name outside the module in which
they are defined, they can be used as a sort of ADT.

src/test/run-pass/export-unexported-dep.rs

+// This tests that exports can have visible dependencies on things
+// that are not exported, allowing for a sort of poor-man's ADT
+
+mod foo {
+  export f;
+  export g;
+
+  // not exported
+  tag t {
+    t1;
+  }
+
+  fn f() -> t {
+    ret t1;
+  }
+
+  fn g(t v) {
+    assert v == t1;
+  }
+}
+
+fn main() {
+  foo.g(foo.f());
+}
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