[clang] clang-format: Add "AllowShortNamespacesOnASingleLine" option (PR #105597)

[Owen Pan via cfe-commits]

https://github.com/owenca commented:

The following are missing:
- Run `dump_format_style.py`.
- Add a `ConfigParseTest` for the new option.

How does the new option interact with `CompactNamespaces`? For example:
- `AllowShortNamespacesOnASingleLine: true` and `CompactNamespaces: false`
```
namespace a {
namespace b { class c; }
} // namespace a
```
- Both `true`
`namespace a { namespace b { class c; } }`

https://github.com/llvm/llvm-project/pull/105597