[libc++] Do not guard inclusion of wchar.h with _LIBCPP_HAS_WIDE_CHARACTERS

mbstate_t needs to be visible to libcpp, even when it is not relying on
wide character functionality in the C library. C90 amendment 1 says that
this type is to be visible through wchar.h. That header file is provided
by the C standard library even when that library is not implementing wchar
functions (such as newlib with --nano-formatted-io).

Since we have a way to conditionally include <wchar.h> only if it exists,
we should rely on the fact that if it exists, it will provide mbstate_t.

Author: stevew817

libcxx/include/__mbstate_t.h

@@ -43,12 +43,12 @@
 #  include <bits/types/mbstate_t.h> // works on most Unixes
 #elif __has_include(<sys/_types/_mbstate_t.h>)
 #  include <sys/_types/_mbstate_t.h> // works on Darwin
-#elif _LIBCPP_HAS_WIDE_CHARACTERS && __has_include_next(<wchar.h>)
-#  include_next <wchar.h> // fall back to the C standard provider of mbstate_t
+#elif __has_include_next(<wchar.h>)
+#  include_next <wchar.h> // user the C standard provider of mbstate_t if present
 #elif __has_include_next(<uchar.h>)
-#  include_next <uchar.h> // <uchar.h> is also required to make mbstate_t visible
+#  include_next <uchar.h> // <uchar.h> can alternatively provide mbstate_t
 #else
-#  error "We don't know how to get the definition of mbstate_t without <wchar.h> on your platform."
+#  error "We don't know how to get the definition of mbstate_t on your platform."
 #endif
 
 #endif // _LIBCPP___MBSTATE_T_H