fix: crash on unimplemented when populating overloads destructor entry

I am not sure why this is not seen in CI presently, but I can reproduce
it when building mrdocs from the llvm version bundled in homebrew
(currently 20.1.7).

Author: mizvekov

src/lib/Metadata/Finalizers/Javadoc/Overloads.hpp

@@ -76,6 +76,10 @@ populateOverloadsFromClass(OverloadsInfo& I)
         return true;
     }
     case FunctionClass::Destructor:
+    {
+        I.javadoc->brief = "Destructors";
+        return true;
+    }
     default:
     MRDOCS_UNREACHABLE();
     }