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

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.

Differential Revision: https://reviews.llvm.org/D119647

Author: mstorsjo

libcxx/src/include/ryu/d2s_intrinsics.h

@@ -44,6 +44,8 @@
 
 #include "__config"
 
+#include "include/ryu/ryu.h"
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if defined(_M_X64) && defined(_MSC_VER)

libcxx/src/include/ryu/ryu.h

@@ -56,9 +56,9 @@
 #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
@@ -68,7 +68,7 @@ _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;
@@ -84,7 +84,7 @@ _LIBCPP_HIDE_FROM_ABI inline unsigned char _BitScanForward(unsigned long* __inde
   *__index = __builtin_ctz(__mask);
   return true;
 }
-#endif  // _LIBCPP_COMPILER_MSVC
+#endif  // !_MSC_VER
 
 template <class _Floating>
 [[nodiscard]] to_chars_result _Floating_to_chars_ryu(