SILGen: Don't try using materializeForSet with storage in an @objc protocol

Fixes <https://bugs.swift.org/browse/SR-2669>.

Author: slavapestov

lib/SILGen/SILGenLValue.cpp

@@ -831,10 +831,11 @@ namespace {
         return true;
       }
 
-      // If the declaration is dynamically dispatched through a protocol,
-      // we have to use materializeForSet.
-      if (isa<ProtocolDecl>(decl->getDeclContext()))
-        return true;
+      // If the declaration is dynamically dispatched through a
+      // non-ObjC protocol, we have to use materializeForSet.
+      if (auto *protoDecl = dyn_cast<ProtocolDecl>(decl->getDeclContext()))
+        if (!protoDecl->isObjC())
+          return true;
 
       return false;
     }

test/SILGen/objc_protocols.swift

@@ -280,3 +280,12 @@ extension InitializableConformerByExtension: Initializable {
   }
 }
 // CHECK-LABEL: sil hidden [thunk] @_TToFC14objc_protocols33InitializableConformerByExtensionc
+
+// Make sure we're crashing from trying to use materializeForSet here.
+@objc protocol SelectionItem {
+  var time: Double { get set }
+}
+
+func incrementTime(contents: SelectionItem) {
+  contents.time += 1.0
+}