Summary:
First of all, seems it's the right thing to do. Fabric C++ code is cross-platfrom and should run on *all* platforms including Windows, Linux, and Mac.
While we don't have a real *production* use cases where we need compilation for desktops, having CXX target is really handy for two reasons:
* It simplifies local test running process. Instead of going to `/fbandroid/` and executing something like `buck test fbsource//xplat/js/react-native-github/ReactCommon/fabric/core:coreAndroid` (note the suffix). We can just do `buck test fbsource//xplat/js/react-native-github/ReactCommon/fabric/core:core` everywhere and it works now out of the box. Running tests with "Apple" flavor never worked for me.
* It allows creating synthetic benchmark tests (using Google Benchmark) that can be used as a rough approximation of code micro-optimizations.
Reviewed By: JoshuaGross
Differential Revision: D15608678
fbshipit-source-id: d2449035685dbca6ab983480f5334ec4ac11cd35
15302284cc