Add test for overlapping pattern warnings for lazy patterns

testsuite/tests/ghc-regress/deSugar/should_compile/all.T

@@ -61,3 +61,4 @@ test('ds054', normal, compile, [''])
 test('ds055', only_compiler_types(['ghc']), compile, [''])
 test('ds056', normal, compile, ['-Wall'])
 test('ds057', normal, compile, [''])
+test('ds058', normal, compile, ['-W'])

testsuite/tests/ghc-regress/deSugar/should_compile/ds058.hs

+-- Test overlapping pattern warnings
+
+module ShouldCompile where
+
+f x = case x of 
+	Just (~1) -> 0
+        Just _    -> 1	-- This one cannot match
+        Nothing   -> 2

testsuite/tests/ghc-regress/deSugar/should_compile/ds058.stderr

+
+ds058.hs:5:6:
+    Warning: Pattern match(es) are overlapped
+	     In a case alternative: Just _ -> ...