swift234

Add support for :print_module on Clang modules.

This isn't very efficient: it scans every decl in the Clang TU (forcing
deserialization) and filters based on the decl's enclosing module.
Moreover, since getClangModuleForDecl() currently only handles top-level
modules, all submodules get implicitly added to the top-level module...
and will /not/ match an explicit submodule request.

(This is probably close to the behavior we actually want: include decls that
are from modules that are (a) submodules and (b) re-exported by the top-level
module. We do want that extra check, though, and we would want to find things
specifically by submodule.)

Swift SVN r7602