Test that having no return type on msg_send doesn't compile

Author: madsmtm

objc2/tests/ui/msg_send_no_return_type.rs

@@ -0,0 +1,8 @@
+use objc2::{class, msg_send};
+
+fn main() {
+    unsafe {
+        let cls = class!(NSObject);
+        let _obj = msg_send![cls, new];
+    }
+}

objc2/tests/ui/msg_send_no_return_type.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+ --> $DIR/msg_send_no_return_type.rs:6:20
+  |
+6 |         let _obj = msg_send![cls, new];
+  |             ----   ^^^^^^^^^^^^^^^^^^^ cannot infer type
+  |             |
+  |             consider giving `_obj` a type
+  |
+  = note: this error originates in the macro `msg_send` (in Nightly builds, run with -Z macro-backtrace for more info)