The first set of patches is in review, please review if you have time:
⚙ D145566 [lldb] Add RegisterFlags class
⚙ D145568 [lldb] Add dummy field to RegisterInfo for register flags use later
⚙ D145574 [lldb] Read register fields from target XML
⚙ D145580 [lldb] Show register fields using bitfield struct types
This allows lldb to read register fields from debug servers like gdbserver that already emit them.
The next stage will be getting lldb and/or lldb-server to generate the same information. I will look into both ways of doing it.