I am experimenting with putting visibility-default attributes on all...

I am experimenting with putting visibility-default attributes on all struct/classes in libc++.  This checkin decorates only basic_string and vector as an experiment, and for review by those in this audience that might know more about visibilty than I do.  If I get no negative feedback on this procedure I will begin to decorate the entire library in this way.

llvm-svn: 113590