[lldb] Cleanup MacOSX platform headers (NFC)

While working on dde487e5 I noticed that the MacOSX platforms were
in need of some love. This patch cleans up the headers:

 - Move platforms into the lldb_private namespace.
 - Remove lldb_private:: prefixes to improve readability.
 - Fix header includes and use forward declarations (iwyu).
 - Fix formatting