[libc++] Keep __regex_word in sync with ctype_base

Summary:
The class ctype_base in the header <__locale> contains masks for
character classification functions, which are kept in sync with
platform's C library, hence it contains many special cases.
The value of the bit mask __regex_word in the header <regex> must not
clash with those bit masks.

Currently the default case (i.e. unknown platform/C library) is
handled incorrectly: the __regex_word clashes with ctype_base::punct.

To avoid replicating the whole list of platforms in <regex> this patch
defines __regex_word in <__locale>, so that it is always kept in sync
with other masks.

Reviewers: ldionne, mclow.lists, EricWF

Reviewed By: ldionne

Subscribers: krytarowski, christof, dexonsmith, pbarrio, simon_tatham, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D63284

llvm-svn: 363363