Merge pull request swiftlang#12190 from graydon/objc-protocols-can-always-be-existential

[ProtocolDecl] Avoid getMembers(), imported ObjC protocols can always be existential.

Author: graydon

lib/AST/Decl.cpp

@@ -3380,6 +3380,10 @@ bool ProtocolDecl::existentialTypeSupportedSlow(LazyResolver *resolver) {
   ProtocolDeclBits.ExistentialTypeSupportedValid = true;
   ProtocolDeclBits.ExistentialTypeSupported = true;
 
+  // ObjC protocols can always be existential.
+  if (isObjC())
+    return true;
+
   // Resolve the protocol's type.
   if (resolver && !hasInterfaceType())
     resolver->resolveDeclSignature(this);