Swift handles Clang submodules as far as controlling visibility, but
pretends that all the declarations live in the top-level module for
the purposes of lookup. This was interacting poorly with another
feature: automatically importing the Swift part of a mixed-source
framework and using that as the "presentation" of a Clang module...
or a submodule in the same framework. Therefore, if you are
- compiling the Swift part of a mixed-source framework, and
- importing one of the framework's (Clang) submodules
name binding would say that the current file imports the Swift module
currently being compiled, in addition to the Clang module we actually
want. Just break the cycle in that case.
