Allow implicit discard when right-hand side is Void.

This is the most intuitive/expected behavior. Otherwise, it forces a pointless let-binding to 'Void' or into a discard.

examples/acceptance_tests/098/lib/foo.ak

@@ -0,0 +1,8 @@
+fn label(str: String) -> Void {
+  trace str Void
+}
+
+test foo() {
+  label(@"Foo")
+  True
+}