Add let_else feature gate

src/test/ui/feature-gates/feature-gate-let_else.rs

@@ -0,0 +1,5 @@
+fn main() {
+    let Some(x) = Some(1) else { //~ ERROR `let...else` statements are unstable
+        return;
+    };
+}

src/test/ui/feature-gates/feature-gate-let_else.stderr

@@ -0,0 +1,14 @@
+error[E0658]: `let...else` statements are unstable
+  --> $DIR/feature-gate-let_else.rs:2:5
+   |
+LL | /     let Some(x) = Some(1) else {
+LL | |         return;
+LL | |     };
+   | |______^
+   |
+   = note: see issue #87335 <https://github.com/rust-lang/rust/issues/87335> for more information
+   = help: add `#![feature(let_else)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.