[clang-format] C feature request: always multi-line un-nested designated initializer

e.g.:
```C++
return (StrView){
.len = end - begin,
.chars = str + begin
};
```
```C++
int *arr = (int[]){
[10] = 2
};
```

adding a trailing comma makes clang-format does this, but I think it makes sense to have an option to always do it like this.

Start by reproducing the two designated-initializer examples with clang-format, both with and without a trailing comma, to confirm the current behavior. Then identify the clang-format formatting entry point and existing tests for initializer formatting, and define the new option's expected output for both examples.

Written by the indexing model from the issue text.