cases: add compile error test for never-mutated local variable

test/cases/compile_errors/var_never_mutated.zig

@@ -0,0 +1,28 @@
+fn entry0() void {
+    var a: u32 = 1 + 2;
+    _ = a;
+}
+
+fn entry1() void {
+    const a: u32 = 1;
+    const b: u32 = 2;
+    var c = a + b;
+    const d = c;
+    _ = d;
+}
+
+fn entry2() void {
+    var a: u32 = 123;
+    foo(a);
+}
+
+fn foo(_: u32) void {}
+
+// error
+//
+// :2:9: error: local variable is never mutated
+// :2:9: note: consider using 'const'
+// :9:9: error: local variable is never mutated
+// :9:9: note: consider using 'const'
+// :15:9: error: local variable is never mutated
+// :15:9: note: consider using 'const'