[ABI] Rework protocol descriptor metadata.

Reimplement protocol descriptors for Swift protocols as a kind of
context descriptor, dropping the Objective-C protocol compatibility
layout. The new protocol descriptors have several advantages over the
current implementation:

* They drop all of the unused fields required for layout-compatibility
  with Objective-C protocols.
* They encode the full requirement signature of the protocol. This
  maintains more information about the protocol itself, including
  (e.g.) correctly encoding superclass requirements.
* They fit within the general scheme of context descriptors, rather than
  being their own thing, which allows us to share more code with
  nominal type descriptors.
* They only use relative pointers, so they’re smaller and can be placed
  in read-only memory

 Implements rdar://problem/38815359.

stdlib/public/runtime/Metadata.cpp

@@ -3339,7 +3339,7 @@ static void initializeResilientWitnessTable(GenericWitnessTable *genericTable,
                                             void **table) {
   auto protocol = genericTable->Protocol.get();
 
-  auto requirements = protocol->Requirements.get();
+  auto requirements = protocol->getRequirements();
   auto witnesses = genericTable->ResilientWitnesses->getWitnesses();
 
   for (size_t i = 0, e = protocol->NumRequirements; i < e; ++i) {