[libcxx] use __builtin_isnan in std::isnan.

[TokarIP]

Summary

This allows std::isnan to be fully inlined, instead of generating calls.

[EricWF]

OK, this LGTM.

The C library header tries to perform this optimization for us, but it fails to detect that Clang has __builtin_isnan because it reports as GCC 4.2, and the C library checks for GCC 4.4.

It's unfortunate we need to do this ourselves, but I'm OK making this change.

[EricWF]

Since the __libcpp_is_nan_or_builtin hacks are no longer needed (they can directly call isnan), could you please send a separate change to remove it?

[EricWF]

Sorry, I meant to LGTM this formally last time.

libcxx/include/math.h

 
 #ifdef isnan
 
 template <class _A1>
 _LIBCPP_INLINE_VISIBILITY
 bool
 __libcpp_isnan(_A1 __lcpp_x) _NOEXCEPT
 {
+#if __has_builtin(__builtin_isnan)
+    return __builtin_isnan(__lcpp_x);
+#else
     return isnan(__lcpp_x);
+#endif
 }
 
 #undef isnan
 
 template <class _A1>
 inline _LIBCPP_INLINE_VISIBILITY
 typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
 isnan(_A1 __lcpp_x) _NOEXCEPT