[clang-tidy] fix-it for performance-inefficient-string-concatenation

performance-inefficient-string-concatenation may benefit from having a fix-it to allow transformations like

```cpp

string1 = string1 + string2;

```

to

```cpp

string1 += string2;

```

and maybe:

```cpp
string1 = string1 + "bar" + string2;
```
to

```cpp
string1.append("bar").append(string2);
```

I don't have the full grasp on which checkers are allowed to have fix-its but this one feels straightforward and side-effect free enough that can be helpful.

https://godbolt.org/z/Wdd3o7sb9