Sema: validate @intToEnum int operand type

test/cases/compile_errors/discarding_error_value.zig

@@ -0,0 +1,12 @@
+export fn entry() void {
+    _ = foo();
+}
+fn foo() !void {
+    return error.OutOfMemory;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:12: error: error is discarded. consider using `try`, `catch`, or `if`