Fix Yoga's conditional use of std::bit_cast<>()

Summary:
Yoga tries to use `std::bit_cast<>()` where it's available and falls back to
`std::memcpy()` everywhere else.  Unfortunately, the feature-test macro
(`__cpp_lib_bit_cast`) is only defined if the feature is available *and you have
already included either `<version>` or `<bit>`* (or something else that includes
one of those).  Since `CompactValue.h` checks `__cpp_lib_bit_cast` *first*, it's
not defined even if it *would be*  defined, leading the header not to
`#include <bit>`, leaving `std::bit_cast<>()` undefined; later, other headers
from the STL are included, leading to `__cpp_lib_bit_cast` *becoming* defined
and causing later code to choose to use the undefind `std::bit_cast<>()`.

This diff fixes the problem by `#include`ing either `<version>` or `<ciso646>`
(depending on availability) before checking any feature-test macros.

Changelog: [Internal]

Reviewed By: smeenai

Differential Revision: D41641205

fbshipit-source-id: 7d7bc5791c902a45302d3707e6cbf21fc0493f0c

yoga/CompactValue.h

@@ -9,6 +9,14 @@
 
 #ifdef __cplusplus
 
+#if defined(__has_include) && __has_include(<version>)
+// needed to be able to evaluate defined(__cpp_lib_bit_cast)
+#include <version>
+#else
+// needed to be able to evaluate defined(__cpp_lib_bit_cast)
+#include <ciso646>
+#endif
+
 #ifdef __cpp_lib_bit_cast
 #include <bit>
 #else