[lldb] Harden register type XML serialization (#218503)
This is patch 1/5 preparing LLDB's register-type infrastructure for GDB
XML vector types.
- Deduplicate emitted type definitions by XML ID instead of object
address.
- Centralize XML attribute escaping in `RegisterType`.
- Escape enum, flags, field-reference, and register type IDs.
- Add missing `&` escaping to LLGS XML attributes.
XML references identify types by their textual ID, so separate objects
with the same ID must not produce duplicate definitions.
Co-authored-by: Bar Soloveychik <barsolo@fb.com>
