[ClangImporter] When importing ObjC pointer types, wrap protocol

compositions in ExistentialType when explicit existentials are
enabled.

Author: hborla

lib/ClangImporter/ImportType.cpp

@@ -1052,6 +1052,9 @@ namespace {
               Type importedTypeArg = ProtocolCompositionType::get(
                   Impl.SwiftContext, memberTypes,
                   hasExplicitAnyObject);
+              if (Impl.SwiftContext.LangOpts.EnableExplicitExistentialTypes) {
+                importedTypeArg = ExistentialType::get(importedTypeArg);
+              }
               importedTypeArgs.push_back(importedTypeArg);
             }
           }
@@ -1203,6 +1206,10 @@ namespace {
         importedType = ProtocolCompositionType::get(Impl.SwiftContext,
                                                     members,
                                                     /*HasExplicitAnyObject=*/false);
+
+        if (Impl.SwiftContext.LangOpts.EnableExplicitExistentialTypes) {
+          importedType = ExistentialType::get(importedType);
+        }
       }
 
       // Class or Class<P> maps to an existential metatype.

test/ClangImporter/objc_bridging_generics.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -parse-as-library -verify -swift-version 4 -I %S/Inputs/custom-modules %s
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -parse-as-library -enable-explicit-existential-types -verify -swift-version 4 -I %S/Inputs/custom-modules %s
 
 // REQUIRES: objc_interop