Adds the runtime implementation for copy-on-write existentials. This support is
enabled if SWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS is defined. Focus is on
correctness -- not performance yet.
Don't use allocate/deallocate/projectBuffer witnesses for globals in cow
existential mode.
Use SWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS configuration to set the default for
SILOptions.
This includes an IRGen fix to use the right projection in
emitMetatypeOfOpaqueExistential if SWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS is set.
Use unknownRetain instead of native retain in dynamicCastToExistential.
MetadataCache's allocator into it.
The major functional change here is that MetadataCache will now use
the slab allocator for tree nodes, but I also switched the Hashable
conformances cache to use ConcurrentMap directly instead of a
Lazy<ConcurrentMap<>>.
Previously AnyHashable would consider SwiftValue to be a subclass of
NSObject (which it is in practice) and return false when trying to
compare an AnyHashable of a SwiftValue box to an AnyHashable of the
unboxed value.
If the Swift error wrapped in a _SwiftNativeNSError box conforms to
Hashable, the box now uses the Swift's conformance to Hashable.
Part of rdar://problem/27574348.
