os_unfair_lock is much smaller than pthread_mutex_t (4 bytes versus 64) and a bit faster.
However, it doesn't support condition variables. Most of our uses of Mutex don't use condition variables, but a few do. Introduce ConditionMutex and StaticConditionMutex, which allow condition variables and continue to use pthread_mutex_t.
On all other platforms, we continue to use the same backing mutex type for both Mutex and ConditionMutex.
rdar://problem/45412121
At the moment CYGWIN pthreads implementation doesn't support the use of constexpr for static allocation versions. The way they define things results in a reinterpret_cast which violates constexpr.
- added read / write lock support
- added non-fatal error support to allow use of mutex in fatal error reporting pathway
- isolated pthread implementation to it own header/cpp file pair
- expanded unit tests to cover new code as well as better test existing mutex
- removed a layer of complexity that added no real value
