[Runtime] Use a prefix Objective-C protocol layout to extract the name.

Introduce TargetObjCProtocolPrefix, which describes just enough of the
Objective-C runtime’s Protocol structure to extract the name without
having to call Objective-C’s protocol_getName().

Author: DougGregor

include/swift/ABI/Metadata.h

@@ -1577,6 +1577,19 @@ using TupleTypeMetadata = TargetTupleTypeMetadata<InProcess>;
 
 template <typename Runtime> struct TargetProtocolDescriptor;
 
+#if SWIFT_OBJC_INTEROP
+/// Layout of a small prefix of an Objective-C protocol, used only to
+/// directly extract the name of the protocol.
+template <typename Runtime>
+struct TargetObjCProtocolPrefix {
+  /// Unused by the Swift runtime.
+  TargetPointer<Runtime, const void> _ObjC_Isa;
+
+  /// The mangled name of the protocol.
+  TargetPointer<Runtime, const char> Name;
+};
+#endif
+
 /// A reference to a protocol within the runtime, which may be either
 /// a Swift protocol or (when Objective-C interoperability is enabled) an
 /// Objective-C protocol.
@@ -1647,7 +1660,14 @@ class TargetProtocolDescriptorRef {
 
   /// The name of the protocol.
   TargetPointer<Runtime, const char> getName() const {
-    return getProtocolDescriptorUnchecked()->Name;
+#if SWIFT_OBJC_INTEROP
+    if (isObjC()) {
+      return reinterpret_cast<TargetObjCProtocolPrefix<Runtime> *>(
+          getObjCProtocol())->Name;
+    }
+#endif
+
+    return getSwiftProtocol()->Name;
   }
 
   /// Determine what kind of protocol this is, Swift or Objective-C.