swift234

Lock down on exports from TranslationUnit modules.

Previously, export control (via [exported]) only applied to modules that
had been serialized -- anything imported in a TranslationUnit was
automatically considered exported. This was done to make life easier when
dealing with the main source file, but it turns out we're going to want to
load other source files as imports, and export control should work there too.

Now, the module iteration methods (Module::forAllVisibleModules,
namelookup::lookupInModule, etc.) can consider the module being passed as
"top-level", meaning its private imports are visible as well. Otherwise,
proper export control is observed even for imported TranslationUnits.

This required a number of test changes involving Swift adapter modules that
forgot to re-export their Clang modules.

Swift SVN r7783