swift234

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