[libcxx] [Windows] Pick a unique bit for __regex_word

The old `__regex_word` aliased the mask for `xdigit`, causing stray
test failures.

The diff may look surprising, as if the previous faulty value had
been set specifically for Windows - but this is due to a restructuring
in 411c630bae. Prior to that, there
were OS specific settings for some OSes, and one fallback used for
the rest (which turns out to not work for Windows).

Differential Revision: https://reviews.llvm.org/D118188

libcxx/include/__locale

@@ -452,7 +452,7 @@ public:
     static const mask punct  = _PUNCT;
     static const mask xdigit = _HEX;
     static const mask blank  = _BLANK;
-    static const mask __regex_word = 0x80;
+    static const mask __regex_word = 0x4000; // 0x8000 and 0x0100 and 0x00ff are used
 # define _LIBCPP_CTYPE_MASK_IS_COMPOSITE_PRINT
 #elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
 # ifdef __APPLE__
@@ -545,6 +545,12 @@ public:
     static const mask graph  = alnum | punct;
 
     _LIBCPP_INLINE_VISIBILITY ctype_base() {}
+
+// TODO: Remove the ifndef when the assert no longer fails on AIX.
+#ifndef _AIX
+    static_assert((__regex_word & ~(space | print | cntrl | upper | lower | alpha | digit | punct | xdigit | blank)) == __regex_word,
+                  "__regex_word can't overlap other bits");
+#endif
 };
 
 template <class _CharT> class _LIBCPP_TEMPLATE_VIS ctype;

libcxx/test/std/re/re.traits/isctype.pass.cpp

@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
 // XFAIL: LIBCXX-AIX-FIXME
 
 // <regex>

libcxx/test/std/re/re.traits/lookup_classname.pass.cpp

@@ -6,7 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
 // XFAIL: LIBCXX-AIX-FIXME
 
 // <regex>