[libc++][NFC] Remove MSVC specific code.

[Mordante]

Summary

Switching __builtin_clzll to __libcpp_clz should work on all
platforms and no longer require MSVC specific code.

libcxx/include/charconv

from_chars_result from_chars(const char* first, const char* last,
                                long double& value,
                                chars_format fmt = chars_format::general);
 
 } // namespace std
 
 */
 
 #include <__availability>
+#include <__bits>
 #include <__config>
 #include <__errc>
 #include <__utility/to_underlying.h>
 #include <cmath> // for log2f
 #include <cstdint>
 #include <cstdlib> // for _LIBCPP_UNREACHABLE
 #include <cstring>
 #include <limits>
static _LIBCPP_CONSTEXPR uint32_t __pow10_32[] = {
     UINT32_C(1000000000),
 };
 
 template <typename _Tp, typename = void>
 struct _LIBCPP_HIDDEN __traits_base
 {
     using type = uint64_t;
 
-#if !defined(_LIBCPP_COMPILER_MSVC)
     static _LIBCPP_INLINE_VISIBILITY int __width(_Tp __v)
     {
-        auto __t = (64 - __builtin_clzll(__v | 1)) * 1233 >> 12;
+        auto __t = (64 - _VSTD::__libcpp_clz(static_cast<type>(__v | 1))) * 1233 >> 12;
         return __t - (__v < __pow10_64[__t]) + 1;
     }
-#endif
 
     _LIBCPP_AVAILABILITY_TO_CHARS
     static _LIBCPP_INLINE_VISIBILITY char* __convert(_Tp __v, char* __p)
     {
         return __u64toa(__v, __p);
     }
 
     static _LIBCPP_INLINE_VISIBILITY decltype(__pow10_64)& __pow() { return __pow10_64; }
 };
 
 template <typename _Tp>
 struct _LIBCPP_HIDDEN
     __traits_base<_Tp, decltype(void(uint32_t{declval<_Tp>()}))>
 {
     using type = uint32_t;
 
-#if !defined(_LIBCPP_COMPILER_MSVC)
     static _LIBCPP_INLINE_VISIBILITY int __width(_Tp __v)
     {
-        auto __t = (32 - __builtin_clz(__v | 1)) * 1233 >> 12;
+        auto __t = (32 - _VSTD::__libcpp_clz(static_cast<type>(__v | 1))) * 1233 >> 12;
         return __t - (__v < __pow10_32[__t]) + 1;
     }
-#endif
 
     _LIBCPP_AVAILABILITY_TO_CHARS
     static _LIBCPP_INLINE_VISIBILITY char* __convert(_Tp __v, char* __p)
     {
         return __u32toa(__v, __p);
     }
 
     static _LIBCPP_INLINE_VISIBILITY decltype(__pow10_32)& __pow() { return __pow10_32; }

 template <typename _Tp>
 _LIBCPP_AVAILABILITY_TO_CHARS
 inline _LIBCPP_INLINE_VISIBILITY to_chars_result
 __to_chars_itoa(char* __first, char* __last, _Tp __value, false_type)
 {
     using __tx = __itoa::__traits<_Tp>;
     auto __diff = __last - __first;
 
-#if !defined(_LIBCPP_COMPILER_MSVC)
     if (__tx::digits <= __diff || __tx::__width(__value) <= __diff)
         return {__tx::__convert(__value, __first), errc(0)};
     else
         return {__last, errc::value_too_large};
-#else
-    if (__tx::digits <= __diff)
-        return {__tx::__convert(__value, __first), {}};
-    else
-    {
-        char __buf[__tx::digits];
-        auto __p = __tx::__convert(__value, __buf);
-        auto __len = __p - __buf;
-        if (__len <= __diff)
-        {
-            _VSTD::memcpy(__first, __buf, __len);
-            return {__first + __len, {}};
-        }
-        else
-            return {__last, errc::value_too_large};
-    }
-#endif
 }
 
 template <typename _Tp>
 _LIBCPP_AVAILABILITY_TO_CHARS
 inline _LIBCPP_INLINE_VISIBILITY to_chars_result
 __to_chars_integral(char* __first, char* __last, _Tp __value, int __base,
                     true_type)
 {