[stdlib] Static stdlib requires explicit -licucore

lib/Driver/ToolChains.cpp

@@ -1240,6 +1240,7 @@ toolchains::Darwin::constructInvocation(const LinkJobAction &job,
     Arguments.push_back("-framework");
     Arguments.push_back("Foundation");
     Arguments.push_back("-force_load_swift_libs");
+    Arguments.push_back("-licucore");
   } else {
     Arguments.push_back(context.Args.MakeArgString(RuntimeLibPath));
   }