let-else: add test for linting unused variables in the first place

+ remove actual unused vars from some other tests to be less distracting

src/test/ui/let-else/let-else-allow-unused.rs

@@ -1,14 +1,15 @@
-// check-pass
 // issue #89807
 
 #![feature(let_else)]
 
 #[deny(unused_variables)]
 
 fn main() {
-    let value = Some(String::new());
+    let value = Some(5);
     #[allow(unused)]
     let banana = 1;
     #[allow(unused)]
     let Some(chaenomeles) = value else { return }; // OK
+    let Some(unused) = value else { return };
+    //~^ ERROR unused variable: `unused`
 }

src/test/ui/let-else/let-else-allow-unused.stderr

@@ -0,0 +1,14 @@
+error: unused variable: `unused`
+  --> $DIR/let-else-allow-unused.rs:13:14
+   |
+LL |     let Some(unused) = value else { return };
+   |              ^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused`
+   |
+note: the lint level is defined here
+  --> $DIR/let-else-allow-unused.rs:5:8
+   |
+LL | #[deny(unused_variables)]
+   |        ^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/let-else/let-else-temp-borrowck.rs

@@ -12,12 +12,12 @@ fn foo<'a>(x: &'a str) -> Result<impl Debug + 'a, ()> {
 
 fn let_else() {
     let x = String::from("Hey");
-    let Ok(s) = foo(&x) else { return };
+    let Ok(_) = foo(&x) else { return };
 }
 
 fn if_let() {
     let x = String::from("Hey");
-    let s = if let Ok(s) = foo(&x) { s } else { return };
+    let _ = if let Ok(s) = foo(&x) { s } else { return };
 }
 
 fn main() {

src/test/ui/let-else/let-else-then-diverge.rs

@@ -1,6 +1,7 @@
-//
 // popped up in in #94012, where an alternative desugaring was
-// causing unreachable code errors
+// causing unreachable code errors, and also we needed to check
+// that the desugaring's generated lints weren't applying to
+// the whole else block.
 
 #![feature(let_else)]
 #![deny(unused_variables)]

src/test/ui/let-else/let-else-then-diverge.stderr

@@ -1,11 +1,11 @@
 error: unused variable: `x`
-  --> $DIR/let-else-then-diverge.rs:11:13
+  --> $DIR/let-else-then-diverge.rs:12:13
    |
 LL |         let x = 5;
    |             ^ help: if this is intentional, prefix it with an underscore: `_x`
    |
 note: the lint level is defined here
-  --> $DIR/let-else-then-diverge.rs:6:9
+  --> $DIR/let-else-then-diverge.rs:7:9
    |
 LL | #![deny(unused_variables)]
    |         ^^^^^^^^^^^^^^^^