Add rustdoc regression test for the unused_braces lint

Author: flip1995

src/test/rustdoc-ui/unused-braces-lint.rs

@@ -0,0 +1,14 @@
+// check-pass
+
+// This tests the bug in #70814, where the unused_braces lint triggered on the following code
+// without providing a span.
+
+#![deny(unused_braces)]
+
+fn main() {
+    {
+        {
+            use std;
+        }
+    }
+}