swift234

Clang importer: centralize the logic for importing instance methods as class methods.

Objective-C instance methods of root classes can also be used as class
methods, which we handle by performing a redundant import, the code
for which was scattered. Centralize this import as part of importing
the original (instance) method, and record it as an alternate
declaration to the original instance method. This eliminates a number
of special cases in the Clang importer.