Rename test and add comment

Author: Aaron1011

src/test/ui/existential-type/issue-52843.rs renamed to src/test/ui/existential-type/issue-52843-closure-constrain.rs

@@ -1,3 +1,4 @@
+// Checks to ensure that we properly detect when a closure constrains an existential type
 #![feature(existential_type)]
 
 use std::fmt::Debug;

src/test/ui/existential-type/issue-52843.stderr renamed to src/test/ui/existential-type/issue-52843-closure-constrain.stderr

@@ -1,11 +1,11 @@
 error: concrete type differs from previous defining existential type use
-  --> $DIR/issue-52843.rs:7:5
+  --> $DIR/issue-52843-closure-constrain.rs:8:5
    |
 LL |     fn _unused() -> Existential { String::new() }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, got `std::string::String`
    |
 note: previous use here
-  --> $DIR/issue-52843.rs:5:1
+  --> $DIR/issue-52843-closure-constrain.rs:6:1
    |
 LL | / fn main() {
 LL | |     existential type Existential: Debug;