yoga | Fix error about implicit conversion to bit-field

Summary:
`YGStyle` puts Yoga enums (which are signed integers by default) into bitfields: https://fburl.com/7fowlunu

Mixing signed values and bit-fields can be error-prone and it also fails to build on Windows with `clang-cl` due to `-Wbitfield-constant-conversion` warning being treated as error:

```
stderr: In file included from xplat\yoga\yoga\YGLayout.cpp:8:
In file included from xplat\yoga\yoga/Utils.h:8:
In file included from xplat\yoga\yoga/YGNode.h:13:
xplat\yoga\yoga/YGStyle.h(110,9): error: implicit truncation from 'YGAlign' to bit-field changes value from 4 to -4 [-Werror,-Wbitfield-constant-conversion]
        alignItems_(YGAlignStretch),
```

This diff fixes the problem by making all enums unsigned integers. This change can be problematic only if values of the enums are serialized somewhere. CC: David Aurelio

Reviewed By: davidaurelio

Differential Revision: D16336729

fbshipit-source-id: ee4dabd7bd1ee429e644bd322b375ec2694cc742

ReactCommon/yoga/yoga/YGStyle.h

@@ -102,6 +102,11 @@ public:
     }
   };
 
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wbitfield-constant-conversion"
+#endif
+
   YGStyle()
       : direction_(YGDirectionInherit),
         flexDirection_(YGFlexDirectionColumn),
@@ -113,6 +118,11 @@ public:
         flexWrap_(YGWrapNoWrap),
         overflow_(YGOverflowVisible),
         display_(YGDisplayFlex) {}
+
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
+
   ~YGStyle() = default;
 
   static constexpr int directionBit = 0;