Simplify bitfields (#1393)

Summary:
X-link: https://github.com/facebook/react-native/pull/39485

Pull Request resolved: https://github.com/facebook/yoga/pull/1393

These were previously packed into structs to allow zero initializing all the flags at once without needing a ctor. C++ 20 has in-class member initializer support for bitfields, which makes these look more like normal member variables.

Setting enum values is a bit jank right now, due to relying on C enums which are effectively int32_t, along with GCC `-Wconversion` being a bit aggressive in needing to explicitly mask. I have some ideas to fix this later (e.g. using scoped enums internally).

bypass-github-export-checks

Changelog:
[General][Breaking] - Require C++ 20 when including renderer headers

Reviewed By: sammy-SC

Differential Revision: D49265967

fbshipit-source-id: 6ab935a866196df06e742c821f3af88eb4d18e1a

yoga/node/LayoutResults.h

@@ -15,14 +15,6 @@
 
 namespace facebook::yoga {
 
-#pragma pack(push)
-#pragma pack(1)
-struct LayoutResultFlags {
-  uint32_t direction : 2;
-  bool hadOverflow : 1;
-};
-#pragma pack(pop)
-
 struct LayoutResults {
   // This value was chosen based on empirical data:
   // 98% of analyzed layouts require less than 8 entries.
@@ -35,7 +27,8 @@ struct LayoutResults {
   std::array<float, 4> padding = {};
 
  private:
-  LayoutResultFlags flags_{};
+  uint32_t direction_ : 2 = static_cast<uint32_t>(YGDirectionInherit) & 0x03;
+  bool hadOverflow_ : 1 = false;
 
  public:
   uint32_t computedFlexBasisGeneration = 0;
@@ -53,18 +46,18 @@ struct LayoutResults {
   CachedMeasurement cachedLayout{};
 
   YGDirection direction() const {
-    return static_cast<YGDirection>(flags_.direction);
+    return static_cast<YGDirection>(direction_);
   }
 
   void setDirection(YGDirection direction) {
-    flags_.direction = static_cast<uint32_t>(direction) & 0x03;
+    direction_ = static_cast<uint32_t>(direction) & 0x03;
   }
 
   bool hadOverflow() const {
-    return flags_.hadOverflow;
+    return hadOverflow_;
   }
   void setHadOverflow(bool hadOverflow) {
-    flags_.hadOverflow = hadOverflow;
+    hadOverflow_ = hadOverflow;
   }
 
   bool operator==(LayoutResults layout) const;