swift234

Implement getReexportedModules for Clang modules.

This handles both Clang’s transitive inclusion and the use of
"adapter modules" to augment the Clang modules (e.g. Foundation.swift),
at the cost of a bit more memory (used to wrap all the Clang modules
in ClangModule objects). This is paving the way for making Sema
independent of ClangImporter.

Swift SVN r6698