Add regression test for #123158

Author: GuillaumeGomez

tests/rustdoc-ui/invalid-redundant-explicit-link.rs

@@ -0,0 +1,8 @@
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/rust/issues/123158>. It
+// should not emit any warning.
+
+//! [**`SomeTrait`**](SomeTrait):
+
+pub trait SomeTrait {}