[clang] bad error message on incorrect string literal #18079 (PR #81670)

[via cfe-commits]

cor3ntin wrote:

It would be better to do, in all cases 
```cpp
 Diag(PrefixEnd, diag::err_invalid_char_raw_delim) <<  escapeCStyle<EscapeChar::Single>(*PrefixEnd);
```

(and remove the `err_invalid_newline_raw_delim` diagnostic altogether) 

https://github.com/llvm/llvm-project/pull/81670