Include non-explicit submodules in exported module list

Include non-explicit submodules in exported module list

This change fixes Richard's testcase for r193815.  Now we include non-explicit
submodules into the list of exports.

The test failed previously because:
- recursive_visibility_a1.inner is not imported (only recursive_visibility_a1 is),
- thus the 'inner' submodule is not showing up in any of the import lists,
- and because of this getExportedModules() is not returning the
  correct module set -- it only considers modules that are imported.

The fix is to make Module::getExportedModules() include non-explicit submodules
into the list of exports.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194018 91177308-0d34-0410-b5e6-96231b3b80d8