[lldb][lldb-server] Enable sending RegisterFlags as XML (#69951)

This adds ToXML methods to encode RegisterFlags and its fields into XML
according to GDB's target XML format:

https://sourceware.org/gdb/onlinedocs/gdb/Target-Description-Format.html#Target-Description-Format

lldb-server does not use libXML to build XML, so this follows the
existing code that uses strings. Indentation is used so the result is
still human readable.

```
<flags id=\"Foo\" size=\"4\">
  <field name=\"abc\" start=\"0\" end=\"0\"/>
</flags>
```

This is used by lldb-server when building target XML, though no one sets
any fields yet. That'll come in a later commit.