Remove mblen(), mbtowc() and wctomb() from the thread-unsafe functions.

[ed]

Summary

Back in r240527 I added a knob to prevent thread-unsafe functions from
being exposed. mblen(), mbtowc() and wctomb() were also added to this
list, as the latest issue of POSIX doesn't require these functions to be
thread-safe.

It turns out that the only circumstance in which these functions are not
thread-safe is in case they are used in combination with state-dependent
character sets (e.g., Shift-JIS). According to Austin Group Bug 708,
these character sets "[...] are mostly a relic of the past and which
were never supported on most POSIX systems".

Though in many cases the use of these functions can be prevented by
using the reentrant counterparts, they are the only functions that allow
you to query whether the locale's character set is state-dependent. This
means that omitting these functions removes actual functionality.

Let's be a bit less pedantic and drop the guards around these functions.

Links:
http://austingroupbugs.net/view.php?id=708
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2037.htm

[ed]

Friendly ping. :-)

libcxx/trunk/include/__config

 #define _LIBCPP_HAS_NO_STDOUT
 #endif
 
 #if defined(__BIONIC__) || defined(__CloudABI__) ||                            \
     defined(_LIBCPP_HAS_MUSL_LIBC)
 #define _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE
 #endif
 
-// Thread-unsafe functions such as strtok(), mbtowc() and localtime()
+// Thread-unsafe functions such as strtok() and localtime()
 // are not available.
 #ifdef __CloudABI__
 #define _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 #endif
 
 #if __has_feature(cxx_atomic) || __has_extension(c_atomic)
 #define _LIBCPP_HAS_C_ATOMIC_IMP
 #elif _GNUC_VER > 407

libcxx/trunk/include/cstdlib

 #ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::llabs;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
 using ::div;
 using ::ldiv;
 #ifndef _LIBCPP_HAS_NO_LONG_LONG
 using ::lldiv;
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 using ::mblen;
 using ::mbtowc;
 using ::wctomb;
-#endif
 using ::mbstowcs;
 using ::wcstombs;
 #ifdef _LIBCPP_HAS_QUICK_EXIT
 using ::at_quick_exit;
 using ::quick_exit;
 #endif
 #ifdef _LIBCPP_HAS_C11_FEATURES
 using ::aligned_alloc;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_CSTDLIB

libcxx/trunk/test/std/depr/depr.c.headers/stdlib_h.pass.cpp

int main()
     static_assert((std::is_same<decltype(labs((long)0)), long>::value), "");
     static_assert((std::is_same<decltype(llabs((long long)0)), long long>::value), "");
     static_assert((std::is_same<decltype(div(0,0)), div_t>::value), "");
     static_assert((std::is_same<decltype(ldiv(0L,0L)), ldiv_t>::value), "");
     static_assert((std::is_same<decltype(lldiv(0LL,0LL)), lldiv_t>::value), "");
     wchar_t* pw = 0;
     const wchar_t* pwc = 0;
     char* pc = 0;
-#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     static_assert((std::is_same<decltype(mblen("",0)), int>::value), "");
     static_assert((std::is_same<decltype(mbtowc(pw,"",0)), int>::value), "");
     static_assert((std::is_same<decltype(wctomb(pc,L' ')), int>::value), "");
-#endif
     static_assert((std::is_same<decltype(mbstowcs(pw,"",0)), size_t>::value), "");
     static_assert((std::is_same<decltype(wcstombs(pc,pwc,0)), size_t>::value), "");
 }

libcxx/trunk/test/std/language.support/support.runtime/cstdlib.pass.cpp

int main()
     static_assert((std::is_same<decltype(std::div(0,0)), std::div_t>::value), "");
     static_assert((std::is_same<decltype(std::div(0L,0L)), std::ldiv_t>::value), "");
     static_assert((std::is_same<decltype(std::div(0LL,0LL)), std::lldiv_t>::value), "");
     static_assert((std::is_same<decltype(std::ldiv(0L,0L)), std::ldiv_t>::value), "");
     static_assert((std::is_same<decltype(std::lldiv(0LL,0LL)), std::lldiv_t>::value), "");
     wchar_t* pw = 0;
     const wchar_t* pwc = 0;
     char* pc = 0;
-#ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     static_assert((std::is_same<decltype(std::mblen("",0)), int>::value), "");
     static_assert((std::is_same<decltype(std::mbtowc(pw,"",0)), int>::value), "");
     static_assert((std::is_same<decltype(std::wctomb(pc,L' ')), int>::value), "");
-#endif
     static_assert((std::is_same<decltype(std::mbstowcs(pw,"",0)), std::size_t>::value), "");
     static_assert((std::is_same<decltype(std::wcstombs(pc,pwc,0)), std::size_t>::value), "");
 }