[lldb] Rename some register type classes (#213684)

Follow up to #196960.

So that when more types are added, the hierarchy is clear.

RegisterType
  -> RegisterTypeEnum
  -> RegisterTypeFlags
  (in future also...)
  -> RegisterTypeUnion
  -> RegisterTypeVector

Renamed and moved the test file as it will cover all the classes derived
from RegisterType.

GitOrigin-RevId: f80ea9af1f29cc202d0b6ba077e79769b6f6184c
25 files changed