Nearly remove the sublocale from the flat locale model's chpl_localeID_t.

[Reviewed by sungeun.]

This nearly removes the sublocale part of the chpl_localeID_t type as
defined by the flat locale model.  We retain a minimal sublocale just to
prevent initialization mismatches with the {0,0} initializers produced
by the compiler.  The file specific changes are as follows.

include/localeModels/flat/chpl-locale-model.h.
  Minimize the sublocale member of chpl_localeID_t down to just one
  bit.  We can't get rid of it entirely, yet, because the compiler
  generates initializers of the form {0,0} for the chpl_localeID_t
  locale members of wide pointers.

include/chpl-gen-includes.h.
  While here, remove the last remnant explicit references to the .node
  and .subloc members of chpl_localeID_t.  Use the (inlined) access
  functions provided by chpl-locale-model.h instead.


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21810 3a8e244f-b0f2-452b-bcba-4c88e055c3ca