Add test for pub let inside function recovery

Author: obeis

tests/ui/parser/pub-let.fixed

@@ -0,0 +1,9 @@
+// run-rustfix
+
+#![allow(unused_variables)]
+
+fn main() {
+    let x = 1;
+    //~^ ERROR visibility `pub` is not followed by an item
+    //~^^ ERROR visibility does not apply to `let` statement
+}

tests/ui/parser/pub-let.rs

@@ -0,0 +1,9 @@
+// run-rustfix
+
+#![allow(unused_variables)]
+
+fn main() {
+    pub let x = 1;
+    //~^ ERROR visibility `pub` is not followed by an item
+    //~^^ ERROR visibility does not apply to `let` statement
+}

tests/ui/parser/pub-let.stderr

@@ -0,0 +1,16 @@
+error: visibility `pub` is not followed by an item
+  --> $DIR/pub-let.rs:6:5
+   |
+LL |     pub let x = 1;
+   |     ^^^ the visibility
+   |
+   = help: you likely meant to define an item, e.g., `pub fn foo() {}`
+
+error: visibility does not apply to `let` statement
+  --> $DIR/pub-let.rs:6:5
+   |
+LL |     pub let x = 1;
+   |     ^^^^^^^ help: remove pub: `let`
+
+error: aborting due to 2 previous errors
+