IRGen: Emit remote reflection metadata for @objc protocols

I noticed even with multi-payload enum layout working, remote
reflection could not understand Set and Dictionary, because of
issues with Objective-C protocols in the standard library that
are used in the representation of those types.

The problem is that we were taking a different code path and
not emitting a field descriptor. A field descriptor is necessary
to differentiate Objective-C, class-bound and opaque protocols
from each other, so that existentials can use the correct
representation.

Author: slavapestov

lib/IRGen/GenMeta.cpp

@@ -5677,6 +5677,9 @@ namespace {
 /// the protocol descriptor, and for ObjC interop, references to the descriptor
 /// that the ObjC runtime uses for uniquing.
 void IRGenModule::emitProtocolDecl(ProtocolDecl *protocol) {
+  // Emit remote reflection metadata for the protocol.
+  emitFieldMetadataRecord(protocol);
+
   // If the protocol is Objective-C-compatible, go through the path that
   // produces an ObjC-compatible protocol_t.
   if (protocol->isObjC()) {
@@ -5706,8 +5709,6 @@ void IRGenModule::emitProtocolDecl(ProtocolDecl *protocol) {
                                                    init->getType()));
   var->setConstant(true);
   var->setInitializer(init);
-
-  emitFieldMetadataRecord(protocol);
 }
 
 /// \brief Load a reference to the protocol descriptor for the given protocol.

test/Reflection/Inputs/ObjectiveCTypes.swift

@@ -19,6 +19,8 @@ public class HasObjCClasses {
   let rect = CGRect(x: 0, y: 1, width: 2, height: 3)
 }
 
+@objc public protocol OP {}
+
 public func closureHasObjCClasses(b: Bundle, c: NSCoding) -> () -> () {
   return { _ = b; _ = c }
 }

test/Reflection/typeref_decoding_objc.swift

@@ -42,6 +42,9 @@
 // CHECK: rect: __C.CGRect
 // CHECK: (struct __C.CGRect)
 
+// CHECK: TypesToReflect.OP
+// CHECK: -----------------
+
 // CHECK: __ObjC.Bundle
 // CHECK: ---------------
 // CHECK: __ObjC.NSURL