Clang should inform users of the possible places to put attributes

Given e.g.
```c++
void func2() {
  [[maybe_unused]] using T = int;
}
```
Clang currently only produces the error `an attribute list cannot appear here`. It would be really nice if Clang could add a note like

```
note: Did you mean to put the attribute here?
[[maybe_unused]] using T = int;
                        ^
```