[lldb] Simplify color logic in (IOHandler)Editline (NFC)

This patch simplifies the color handling logic in Editline and
IOHandlerEditline:

 - Remove the m_color_prompts property from Editline and use the prompt
   ANSI prefix and suffix as the single source of truth. This avoids
   having to redraw the prompt unnecessarily, for example when colors
   are enabled but the prompt prefix and suffix are empty.

 - Rename m_color_prompts to just m_color in IOHandlerEditline and use
   it to ensure consistency between colored prompts and colored
   auto-suggestions. Some IOHandler explicitly turn off colors (such as
   IOHandlerConfirm) and it doesn't really make sense to have one or the
   other.