Add a mode in which access control is respected for module-scope lookups.

This applies to both qualified and unqualified lookups, and is controlled
by the -enable-access-control and -disable-access-control flags. I've
included both so that -disable-access-control can be put into specific tests
that will eventually need to bypass access control (e.g. stdlib unit tests).
The default is still -disable-access-control.

Swift SVN r19146

lib/Frontend/CompilerInvocation.cpp

@@ -540,6 +540,12 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableCharacterLiterals |= Args.hasArg(OPT_enable_character_literals);
 
+  if (auto A = Args.getLastArg(OPT_enable_access_control,
+                               OPT_disable_access_control)) {
+    Opts.EnableAccessControl
+      = A->getOption().matches(OPT_enable_access_control);
+  }
+
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
 
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);