- Use the name for the cached module, so that we don't end up with a
zillion "x86_64-XXXXXXXX.swiftmodule" files in the cache when we're
working with architecture-specific swiftmodules.
- Diagnose if the expected name is different from the name specified
in the swiftinterface.
- Emit all diagnostics at the location of the import, instead of
without any location at all.
