[cxx-interop] Fix namespace lookup
[cxx-interop] Fix namespace lookup

If a namespace has redecls in different modules, Swift sometimes picked a redecl which is not visible from the requested module, and wrongly removed the namespace from lookup results.

This resulted in errors like this:
```
*** DESERIALIZATION FAILURE ***
Could not deserialize type for 'init(cxxString:)'
Caused by: top-level value not found (std)
Cross-reference to module '__ObjC'
... std
```

ahoppen/swift,glessard/swift,glessard/swift,roambotics/swift,JGiola/swift,roambotics/swift,ahoppen/swift,ahoppen/swift,benlangmuir/swift,JGiola/swift,apple/swift,JGiola/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,roambotics/swift,ahoppen/swift,apple/swift,benlangmuir/swift,ahoppen/swift,JGiola/swift,apple/swift,JGiola/swift,benlangmuir/swift,glessard/swift,roambotics/swift,apple/swift,roambotics/swift,JGiola/swift,roambotics/swift,benlangmuir/swift,apple/swift,ahoppen/swift,glessard/swift,apple/swift,glessard/swift