Add feature gate test for cfg'd out super let.

Author: m-ou-se

tests/ui/feature-gates/feature-gate-super-let.rs

@@ -2,3 +2,10 @@ fn main() {
     super let a = 1;
     //~^ ERROR `super let` is experimental
 }
+
+// Check that it also isn't accepted in cfg'd out code.
+#[cfg(any())]
+fn a() {
+    super let a = 1;
+    //~^ ERROR `super let` is experimental
+}

tests/ui/feature-gates/feature-gate-super-let.stderr

@@ -8,6 +8,16 @@ LL |     super let a = 1;
    = help: add `#![feature(super_let)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 1 previous error
+error[E0658]: `super let` is experimental
+  --> $DIR/feature-gate-super-let.rs:9:5
+   |
+LL |     super let a = 1;
+   |     ^^^^^
+   |
+   = note: see issue #139076 <https://github.com/rust-lang/rust/issues/139076> for more information
+   = help: add `#![feature(super_let)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.