swift234

Clean up KnownIdentifiers.def.

- Remove unused names.
- Define IDENTIFIER in terms of IDENTIFIER_WITH_NAME.
- Adjust each name to always match the corresponding value in case.
- Add an IDENTIFIER_ macro for the common case of defining an underscored name.
- Avoid creating names with double underscores, which are technically reserved
  by the C++ standard.

There are two special cases I left in here for the identifiers '_code' and
'_domain'. I didn't want to call these simply 'Id_code' and 'Id_domain' for
fear someone would try to use them as 'code' and 'domain', so I made them into
'Id_code_' and 'Id_domain_' for now.

No intended functionality change.

Swift SVN r29291