[clang-format] Option to align multi-line comment lines with the `*` character

```c
/* This is a multiline comment,
* and this is the next line.
*/
```

This comment should be formatted into:
```c
/* This is a multiline comment,
* and this is the next line.
*/
```

There is an option ([`AlignTrailingComments`](https://clang.llvm.org/docs/ClangFormatStyleOptions.html#aligntrailingcomments)) that does something similar, but it only works with single-line comments.

Start by locating clang-format's handling of multiline comments and its existing formatting tests. Compare the current output with the requested insertion of a space before each continued line's `*`; done means the shown comment is formatted as requested without disrupting other multiline-comment behavior.

Written by the indexing model from the issue text.