User-defined literals for `YGValue`

Summary:
@public

Adds `_pt` and `_percent` user defined literals to create `YGValue` instances.

This allows to create `YGValue`s in the following form:

```
use namespace facebook::yoga::literals;

auto a = 123_pt; // == YGValue{123.0f, YGUnitPoint}
auto b = -12.5_percent; // == YGValue{-12.5f, YGUnitPercent}
```

Reviewed By: SidharthGuglani

Differential Revision: D13942100

fbshipit-source-id: ce1e2f9431c3e2a99c6e11896a712539cc535e0d

ReactCommon/yoga/yoga/YGValue.h

@@ -54,4 +54,30 @@ inline bool operator!=(const YGValue& lhs, const YGValue& rhs) {
   return !(lhs == rhs);
 }
 
+inline YGValue operator-(const YGValue& value) {
+  return {-value.value, value.unit};
+}
+
+namespace facebook {
+namespace yoga {
+namespace literals {
+
+inline YGValue operator"" _pt(long double value) {
+  return YGValue{static_cast<float>(value), YGUnitPoint};
+}
+inline YGValue operator"" _pt(unsigned long long value) {
+  return operator"" _pt(static_cast<long double>(value));
+}
+
+inline YGValue operator"" _percent(long double value) {
+  return YGValue{static_cast<float>(value), YGUnitPercent};
+}
+inline YGValue operator"" _percent(unsigned long long value) {
+  return operator"" _percent(static_cast<long double>(value));
+}
+
+} // namespace literals
+} // namespace yoga
+} // namespace facebook
+
 #endif