Enable -Wconversion (#1359)

Summary:
X-link: https://github.com/facebook/react-native/pull/39291

Pull Request resolved: https://github.com/facebook/yoga/pull/1359

This enables clang warnings around potentially unsafe conversions, such as those with mismatched signedness, or ones which may lead to truncation.

This should catch issues in local development which create errors for MSVC (e.g. Dash), who's default `/W3` includes warnings akin to `-Wshorten-64-to-32`.

This full set of warnings here is a tad spammy, but probably more useful than not.

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D48954777

fbshipit-source-id: 1ccc07b99d09d1c2d428158149698ffd04025605

yoga/node/LayoutResults.h

@@ -15,6 +15,14 @@
 
 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.
@@ -27,10 +35,7 @@ struct LayoutResults {
   std::array<float, 4> padding = {};
 
 private:
-  static constexpr size_t directionOffset = 0;
-  static constexpr size_t hadOverflowOffset =
-      directionOffset + minimumBitCount<YGDirection>();
-  uint8_t flags = 0;
+  LayoutResultFlags flags_{};
 
 public:
   uint32_t computedFlexBasisGeneration = 0;
@@ -48,17 +53,15 @@ public:
   CachedMeasurement cachedLayout{};
 
   YGDirection direction() const {
-    return getEnumData<YGDirection>(flags, directionOffset);
+    return static_cast<YGDirection>(flags_.direction);
   }
 
   void setDirection(YGDirection direction) {
-    setEnumData<YGDirection>(flags, directionOffset, direction);
+    flags_.direction = static_cast<uint32_t>(direction) & 0x03;
   }
 
-  bool hadOverflow() const { return getBooleanData(flags, hadOverflowOffset); }
-  void setHadOverflow(bool hadOverflow) {
-    setBooleanData(flags, hadOverflowOffset, hadOverflow);
-  }
+  bool hadOverflow() const { return flags_.hadOverflow; }
+  void setHadOverflow(bool hadOverflow) { flags_.hadOverflow = hadOverflow; }
 
   bool operator==(LayoutResults layout) const;
   bool operator!=(LayoutResults layout) const { return !(*this == layout); }