Add test for principal-less cast

Author: Darksonn

tests/ui/cast/ptr-to-ptr-principalless.rs

@@ -0,0 +1,8 @@
+//@ check-pass
+// Test cases involving principal-less traits (dyn Send without a primary trait).
+
+fn lifetime_cast_send<'a, 'b>(a: *mut (dyn Send + 'a)) -> *mut (dyn Send + 'b) {
+    a as _
+}
+
+fn main() {}