Make YGValue.h compile with Clang on Windows

Summary:
- We use a fork of Microsoft's react-native-windows which uses a fork of Facebook's react-native
- YGValue.h does not compile with Clang on Windows
- This change should fix that
- I want to put the change here so that it bubbles back to our fork > https://our.intern.facebook.com/intern/diff/D19656093/

#Changelog:
[General][Fixed] Make YGValue.h compile with Clang on Windows

Reviewed By: SidharthGuglani

Differential Revision: D19717489

fbshipit-source-id: ad867ecaf910bb64a777a06c656a1867bb15484b

Author: christophpurrer

ReactCommon/yoga/yoga/YGValue.h

@@ -11,6 +11,13 @@
 #include "YGEnums.h"
 #include "YGMacros.h"
 
+#if defined(_MSC_VER) && defined(__clang__)
+#define COMPILING_WITH_CLANG_ON_WINDOWS
+#endif
+#if defined(COMPILING_WITH_CLANG_ON_WINDOWS)
+#include <limits>
+constexpr float YGUndefined = std::numeric_limits<float>::quiet_NaN();
+#else
 YG_EXTERN_C_BEGIN
 
 // Not defined in MSVC++
@@ -20,6 +27,7 @@ static const uint32_t __nan = 0x7fc00000;
 #endif
 
 #define YGUndefined NAN
+#endif
 
 typedef struct YGValue {
   float value;
@@ -30,7 +38,10 @@ YOGA_EXPORT extern const YGValue YGValueAuto;
 YOGA_EXPORT extern const YGValue YGValueUndefined;
 YOGA_EXPORT extern const YGValue YGValueZero;
 
+#if !defined(COMPILING_WITH_CLANG_ON_WINDOWS)
 YG_EXTERN_C_END
+#endif
+#undef COMPILING_WITH_CLANG_ON_WINDOWS
 
 #ifdef __cplusplus