Revert "Add a flag -ignore-overlays to ignore overlays, and use it when building the standard library."

This reverts commit r16383.

Swift SVN r16475

lib/Frontend/CompilerInvocation.cpp

@@ -566,7 +566,6 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts, ArgList &Args,
 
   Opts.InferImplicitProperties =
     Args.hasArg(OPT_enable_objc_implicit_properties);
-  Opts.IgnoreAdapterModules = Args.hasArg(OPT_ignore_overlays);
 
   return false;
 }