swift234

Support importing multiple Clang modules into Swift.

This is mostly a hack to work around differences between how Swift and
Clang name lookup into modules works. However, it allows us to load
multiple Clang modules into Swift without causing spurious
ambiguities. The generation-based versioning isn't stricly necessary,
since module imports are resolved up front. However, we may eventually
want to speculatively load modules as part of name binding or type
checking, in which case we'd rather not have stale caches. And it
costs us very little.


Swift SVN r3269