Clang modules: collect exports recursively

This change makes Module::buildVisibleModulesCache() collect exported modules
recursively.

While computing a set of exports, getExportedModules() iterates over the set of
imported modules and filters it.  But it does not consider the set of exports
of those modules -- it is the responsibility of the caller to do this.

Here is a certain instance of this issue.  Module::isModuleVisible says that
CoreFoundation.CFArray submodule is not visible from Cocoa.  Why?

- Cocoa imports Foundation.
- Foundation has an export restriction: "export *".
- Foundation imports CoreFoundation.  (Just the top-level module.)
- CoreFoundation exports CoreFoundation.CFArray.

To decide which modules are visible from Cocoa, we collect all exported modules
from immediate imports in Cocoa:

> visibleModulesFro(Cocoa) = exported(Foundation) + exported(CoreData) + exported(AppKit)

To find out which modules are exported, we filter imports according to
restrictions:

> exported(Foundation) = filterByModuleMapRestrictions(imports(Foundation))

Because Foundation imports CoreFoundation (not CoreFoundation.CFArray), the
CFArray submodule is considered not exported from Foundation, and is not
visible from Cocoa (according to Module::isModuleVisible).