[libcxx][test] Fix numpunct grouping tests on AIX (llvm#91781)

The `grouping` string for locale `en_US.UTF-8` and `fr_FR.UTF-8` on AIX
is `3`. This is different from Linux's `3;3` but is the same as Windows.
This patch removes `XFAIL: LIBCXX-AIX-FIXME` and changes to use the
`WIN32` code path.

libcxx/test/std/localization/locale.categories/facet.numpunct/locale.numpunct.byname/grouping.pass.cpp

@@ -9,7 +9,6 @@
 // NetBSD does not support LC_NUMERIC at the moment
 // XFAIL: netbsd
 
-// XFAIL: LIBCXX-AIX-FIXME
 // XFAIL: LIBCXX-FREEBSD-FIXME
 
 // REQUIRES: locale.en_US.UTF-8
@@ -49,7 +48,7 @@ int main(int, char**)
         {
             typedef char C;
             const std::numpunct<C>& np = std::use_facet<std::numpunct<C> >(l);
-#ifdef _WIN32
+#if defined(_WIN32) || defined(_AIX)
             assert(np.grouping() == "\3");
 #else
             assert(np.grouping() == "\3\3");
@@ -59,17 +58,17 @@ int main(int, char**)
         {
             typedef wchar_t C;
             const std::numpunct<C>& np = std::use_facet<std::numpunct<C> >(l);
-#ifdef _WIN32
+#  if defined(_WIN32) || defined(_AIX)
             assert(np.grouping() == "\3");
-#else
+#  else
             assert(np.grouping() == "\3\3");
-#endif
+#  endif
         }
 #endif
     }
     {
         std::locale l(LOCALE_fr_FR_UTF_8);
-#if defined(TEST_HAS_GLIBC) || defined(_WIN32)
+#if defined(TEST_HAS_GLIBC) || defined(_WIN32) || defined(_AIX)
         const char* const group = "\3";
 #else
         const char* const group = "\x7f";