If a bridging header imported a submodule but /not/ the top-level
module, the top-level module would never be made visible. This is a
problem since the Clang importer makes all top-level decls available
through their enclosing top-level modules, not the individual
submodules.
To fix this, stop special-casing the way we handle bridging header
imports; we can translate them from Clang Modules to Swift ModuleDecls
the same way we do for regular imports.
rdar://problem/37355259
