[Swiftify] Always skip safe wrappers when building the stdlib

_SwiftifyImport assumes types like Swift.Int, Swift.UnsafePointer<T> and
Swift.Span<T> are available. This is not the case when building the
stdlib itself. Disable safe interop in the stdlib to prevent errors.
This currently has no effect, but will when this feature is enabled by
default, which I have manually tested.

lib/Frontend/CompilerInvocation.cpp

@@ -2175,6 +2175,8 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts, ArgList &Args,
 
   Opts.LoadVersionIndependentAPINotes |= Args.hasArg(OPT_version_independent_apinotes);
 
+  Opts.DisableSafeInteropWrappers |= FrontendOpts.ParseStdlib;
+
   if (FrontendOpts.DisableImplicitModules)
     Opts.DisableImplicitClangModules = true;