Add YGErrata Enum (#1256)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1256

X-link: https://github.com/facebook/react-native/pull/37076

This adds a `YGErrata` bitset enum matching the API and guarantees described in https://github.com/facebook/yoga/issues/1247.

It is hooked up in later diffs. There are a couple of `YGExperimentalFeature` values that belong here, but keeping the current options means that the default `YGErrataNone` corresponds to existing default behavior, letting us stage the series of changes as:
1. Implement errata API
2. Update internal Yoga users we want to de-risk to `YGErrataClassic` or `YGErrataAll` (if setting `UseLegacyStretchBehaviour`)
3. Add new errata, changing Yoga defaults to be conformant, while letting internal apps opt into compatibility modes pending experimentation.

I also added a macro to let C++ users of Yoga perform bitwise operations on the enum without casting (already available for C users).

Reviewed By: rshest

Differential Revision: D45254098

fbshipit-source-id: d4b61271a8018f548f2d9d8c953db4b121a502d1

enums.py

@@ -59,6 +59,19 @@ ENUMS = {
     ],
     "PrintOptions": [("Layout", 1), ("Style", 2), ("Children", 4)],
     "Gutter": ["Column", "Row", "All"],
+    # Known incorrect behavior which can be enabled for compatibility
+    "Errata": [
+        # Default: Standards conformant mode
+        ("None", 0),
+        # Allows main-axis flex basis to be stretched without flexGrow being
+        # set (previously referred to as "UseLegacyStretchBehaviour")
+        ("StretchFlexBasis", 1 << 0),
+        # Enable all incorrect behavior (preserve compatibility)
+        ("All", 0x7FFFFFFF),
+        # Enable all errata except for "StretchFlexBasis" (Defaults behavior
+        # before Yoga 2.0)
+        ("Classic", 0x7FFFFFFF & (~(1 << 0))),
+    ],
 }
 
 # Generated Java enums used to emit @DoNotStrip, but D17519844 removed them
@@ -66,6 +79,8 @@ ENUMS = {
 # well?
 DO_NOT_STRIP = ["LogLevel"]
 
+BITSET_ENUMS = ["Errata"]
+
 
 def get_license(ext):
     prologue = "/**" if ext == "js" else "/*"
@@ -132,6 +147,8 @@ with open(root + "/yoga/YGEnums.h", "w") as f:
                 f.write(")\n")
             else:
                 f.write(",\n")
+        if name in BITSET_ENUMS:
+            f.write("YG_DEFINE_ENUM_FLAG_OPERATORS(YG%s)\n" % name)
         f.write("\n")
     f.write("YG_EXTERN_C_END\n")