Xcode: Port rudimentary Swift support to Xcode 8

The `.pbxproj` file must now specify a `SWIFT_VERSION` value.
Set it to the legacy value of "2.3" for now.  Later this can
be made configurable (e.g. to "3.0").

Modules/CMakeDetermineCompilerId.cmake

@@ -266,6 +266,11 @@ Id flags: ${testflags}
     else()
       set(id_toolset "")
     endif()
+    if("${lang}" STREQUAL "Swift")
+      set(id_lang_version "SWIFT_VERSION = 2.3;")
+    else()
+      set(id_lang_version "")
+    endif()
     if(CMAKE_OSX_DEPLOYMENT_TARGET)
       set(id_deployment_target
         "MACOSX_DEPLOYMENT_TARGET = \"${CMAKE_OSX_DEPLOYMENT_TARGET}\";")

Modules/CompilerId/Xcode-3.pbxproj.in

@@ -84,6 +84,7 @@
 				CONFIGURATION_BUILD_DIR = "$(BUILD_DIR)";
 				SYMROOT = .;
 				@id_toolset@
+				@id_lang_version@
 				@id_deployment_target@
 				@id_sdkroot@
 			};

Source/cmGlobalXCodeGenerator.cxx

@@ -2996,6 +2996,9 @@ bool cmGlobalXCodeGenerator::CreateXCodeObjects(
     buildSettings->AddAttribute("GCC_VERSION",
                                 this->CreateString(this->GeneratorToolset));
   }
+  if (this->GetLanguageEnabled("Swift")) {
+    buildSettings->AddAttribute("SWIFT_VERSION", this->CreateString("2.3"));
+  }
 
   std::string symroot = root->GetCurrentBinaryDirectory();
   symroot += "/build";