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

Fixes <rdar://problem/16603168>. This was way too much engineering
effort to ban fork() and vfork().

Swift SVN r16383

lib/Frontend/CompilerInvocation.cpp

@@ -566,6 +566,7 @@ 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;
 }