[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/ProtocolConformance.cpp

@@ -32,22 +32,9 @@ using namespace swift;
 
 #ifndef NDEBUG
 template <> void ProtocolDescriptor::dump() const {
-  unsigned NumInheritedProtocols =
-      InheritedProtocols ? InheritedProtocols->NumProtocols : 0;
-
   printf("TargetProtocolDescriptor.\n"
-         "Name: \"%s\".\n"
-         "ObjC Isa: %p.\n",
-         Name, _ObjC_Isa);
-  Flags.dump();
-  printf("Has Inherited Protocols: %s.\n",
-         (NumInheritedProtocols ? "true" : "false"));
-  if (NumInheritedProtocols) {
-    printf("Inherited Protocol List:\n");
-    for (unsigned i = 0, e = NumInheritedProtocols; i != e; ++i) {
-      printf("%s\n", (*InheritedProtocols)[i]->Name);
-    }
-  }
+         "Name: \"%s\".\n",
+         Name.get());
 }
 
 void ProtocolDescriptorFlags::dump() const {