[libcxx] Fix setup of MSVC specific intrinsics in Ryu code

[mstorsjo]

Summary

This fixes warnings about implicitly declared _umul128 and
__shiftright128 when building for x86_64 with clang-cl.

Use _MSC_VER instead of _LIBCPP_COMPILER_MSVC for enabling MSVC
specific code; _MSC_VER is defined both in clang-cl and MSVC,
while _LIBCPP_COMPILER_MSVC only is defined if building with the
actual MSVC compiler.

Include ryu.h at the head of d2s_intrinsics.h, as it uses
the _LIBCPP_64_BIT define, which is defined in ryu.h.

Now the Ryu files build without warnings with clang-cl for i386,
x86_64, arm and aarch64.

[Mordante]

Thanks for the fixes, LGTM!

libcxx/src/include/ryu/d2s_intrinsics.h

 #ifndef _LIBCPP_SRC_INCLUDE_RYU_DS2_INTRINSICS_H
 #define _LIBCPP_SRC_INCLUDE_RYU_DS2_INTRINSICS_H
 
 // Avoid formatting to keep the changes with the original code minimal.
 // clang-format off
 
 #include "__config"
 
+#include "include/ryu/ryu.h"
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if defined(_M_X64) && defined(_MSC_VER)
 #define _LIBCPP_INTRINSIC128 1
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline uint64_t __ryu_umul128(const uint64_t __a, const uint64_t __b, uint64_t* const __productHi) {
   return _umul128(__a, __b, __productHi);
 }
 

libcxx/src/include/ryu/ryu.h

 #include "__errc"
 #include "cstdint"
 #include "cstring"
 #include "type_traits"
 #include "include/ryu/f2s.h"
 #include "include/ryu/d2s.h"
 #include "include/ryu/d2fixed.h"
 
-#if defined(_M_X64) && defined(_LIBCPP_COMPILER_MSVC)
-#include <intrin0.h> // for _umul128() and __shiftright128()
-#endif // defined(_M_X64) && defined(_LIBCPP_COMPILER_MSVC)
+#if defined(_MSC_VER)
+#include <intrin.h> // for _umul128(), __shiftright128(), _BitScanForward{,64}
+#endif // defined(_MSC_VER)
 
 #if defined(_WIN64) || defined(_M_AMD64) || defined(__x86_64__) ||  defined(__aarch64__)
 #define _LIBCPP_64_BIT
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 // https://github.com/ulfjack/ryu/tree/59661c3/ryu
 
-#if !defined(_LIBCPP_COMPILER_MSVC)
+#if !defined(_MSC_VER)
 _LIBCPP_HIDE_FROM_ABI inline unsigned char _BitScanForward64(unsigned long* __index, unsigned long long __mask) {
   if (__mask == 0) {
     return false;
   }
   *__index = __builtin_ctzll(__mask);
   return true;
 }
 
 _LIBCPP_HIDE_FROM_ABI inline unsigned char _BitScanForward(unsigned long* __index, unsigned int __mask) {
   if (__mask == 0) {
     return false;
   }
   *__index = __builtin_ctz(__mask);
   return true;
 }
-#endif  // _LIBCPP_COMPILER_MSVC
+#endif  // !_MSC_VER
 
 template <class _Floating>
 [[nodiscard]] to_chars_result _Floating_to_chars_ryu(
     char* const _First, char* const _Last, const _Floating _Value, const chars_format _Fmt) noexcept {
     if constexpr (_IsSame<_Floating, float>::value) {
         return __f2s_buffered_n(_First, _Last, _Value, _Fmt);
     } else {
         return __d2s_buffered_n(_First, _Last, _Value, _Fmt);