warning diagnostics for unused locals

examples/programs/unused_locals/main.bro

@@ -0,0 +1,12 @@
+warn_only func(value i32, unused i32) i32 {
+	local i32 = 1
+	write_only i32 = 2
+	write_only = 3
+	consumed i32 = value
+	_ = consumed
+	return value
+}
+
+main func() i32 {
+	return warn_only(7, 9)
+}