...since <http://llvm.org/viewvc/llvm-project?view=revision&revision=301647>
"Use the -Wunknown-warning-option group for the 'unknown warning group'
diagnostic in #pragma diagnostic".

* external/boost/include/boost/{locale.hpp,locale/gnu_gettext.hpp} would have
  been removed by 'make cmd cmd=bin/gen-boost-headers' as they are still unused
  from c25eee44 "build boost::locale library",
  but there's reportedly a patch in Gerrit to actually use them, so I fixed them
  manually for now.

* The deviating comment style is to keep lines no wider than 80 characters.

Change-Id: I64603ae8d8a82781eda46f12c9dd5c68dcf395b9