Target C++ 17 (#1327)

Summary:
X-link: https://github.com/facebook/react-native/pull/38303

Pull Request resolved: https://github.com/facebook/yoga/pull/1327

This bumps Yoga to C++ 17 for a few reasons:
1. New versions of C++ may introduce behavior changes (e.g. evaluation order) and deprecations. Keeping the version closer to the version of large users helps avoid that.
2. C++ 17 unblocks some new bits I have wanted to use at times, like `std::optional`, `std::variant`, `if constexpr`, `[[nodiscard]]`.
3. There are already changes in C++ 20 that would be directly useful to Yoga, like `std::bit_cast` to avoid `memcpy` style type punning.

There has been some contention around C++ versions before, but by the time the next stable version of Yoga is out, it will have been more than 6 years (~2 C++ versions) since a stable version of Clang/LLVM with C++ 17 support. I would not like to go back further than n-2.

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D47383922

fbshipit-source-id: eb95d4853f2168b68d6df5fddb797236eac55870

Yoga.podspec

@@ -32,7 +32,7 @@ Pod::Spec.new do |spec|
       '-Wall',
       '-Werror',
       '-Wextra',
-      '-std=c++14',
+      '-std=c++17',
       '-fPIC'
   ]
   spec.source_files = 'yoga/**/*.{h,cpp}'