[clang-tidy] detect unnecessary escaping in char/string literals

```cpp
char c = '\"';
```

There is no need to escape the `"` here.

Start by locating the clang-tidy check responsible for inspecting C++ character and string literals, then review its existing tests or test conventions. The work is done when unnecessary escaping such as \" inside a character literal is diagnosed consistently without flagging escapes that are required.

Written by the indexing model from the issue text.