swift234

[Clang importer] Rely on Clang's mapping of API notes to attributes directly.

Rather than managing API notes directly in the Clang importer, tell
Clang where to find the API notes files and let it map them directly
to attributes. Then, use the existing attribute-mapping
mechanisms---with minor extensions for cases where Clang previously
didn't map some Swift-specific API notes---to get at that information.

There is a minor regression where we can no longer apply API notes to
declarations that don't have a specific Clang declaration. The
specific example (NSError's inherited init()) comes from
rdar://problem/21042412; we've regressed here, but the correct fix is
(and has always been) on the Objective-C side
(rdar://problem/19977891).

Finishes rdar://problem/24447420.