Clang importer: use "alternate declarations" when enumerating imported members.
Clang importer: use "alternate declarations" when enumerating imported members.

When we are creating the complete list of imported members for an
Objective-C class or protocol, start relying on the "alternate
declarations" mapping to get the extra members. Do this only for
subscripts at the moment (now that they record alternate declarations
properly and are idempotent), but eventually we should expand this
out.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift