[libc++] Add std::fpclassify overloads for floating-point.

Standard says that implementation of math functions that have floating-point-type parameter should provide an "overload for each cv-unqualified floating-point type".

Author: tolikmalibroda

libcxx/include/math.h

@@ -387,8 +387,16 @@ namespace __math {
 
 // fpclassify
 
-template <class _A1, std::__enable_if_t<std::is_floating_point<_A1>::value, int> = 0>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI int fpclassify(_A1 __x) _NOEXCEPT {
+_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI int fpclassify(float __x) _NOEXCEPT {
+  return __builtin_fpclassify(FP_NAN, FP_INFINITE, FP_NORMAL, FP_SUBNORMAL, FP_ZERO, __x);
+}
+
+template <class = int>
+_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI int fpclassify(double __x) _NOEXCEPT {
+  return __builtin_fpclassify(FP_NAN, FP_INFINITE, FP_NORMAL, FP_SUBNORMAL, FP_ZERO, __x);
+}
+
+_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI int fpclassify(long double __x) _NOEXCEPT {
   return __builtin_fpclassify(FP_NAN, FP_INFINITE, FP_NORMAL, FP_SUBNORMAL, FP_ZERO, __x);
 }