swift234

Clang importer: do selector matches to more carefully wire up overrides.

We were depending on the (nearly) 1-1 mapping from Objective-C
selectors into Swift names when wiring up overrides of Objective-C
methods, but this mapping falls apart under
-enable-omit-needless-words. Use the Objective-C selector, which
represents the truth of overriding in Objective-C, to wire up
overrides properly.