C++ style enums 9/N: FlexDirection (#1394)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1394

X-link: https://github.com/facebook/react-native/pull/39484

Moves internal usages of YGDirection to Direction.

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Changelog: [Internal]

Reviewed By: rshest

Differential Revision: D49336066

fbshipit-source-id: b49e1ffcd79a427e36ea8d2c26debaa98ef3e4c9

yoga/debug/NodeToString.cpp

@@ -142,9 +142,7 @@ void nodeToString(
     const auto& style = node->getStyle();
     if (style.flexDirection() != yoga::Node{}.getStyle().flexDirection()) {
       appendFormattedString(
-          str,
-          "flex-direction: %s; ",
-          YGFlexDirectionToString(style.flexDirection()));
+          str, "flex-direction: %s; ", toString(style.flexDirection()));
     }
     if (style.justifyContent() != yoga::Node{}.getStyle().justifyContent()) {
       appendFormattedString(