swift234

[ClangImporter] Refuse to import a subscript if the index type doesn't match.

If the getter and setter have different index types, there's not much we can
do in Swift. It's better to not pretend to handle this (which currently leads
to a crash in SIL verification). Fortunately, this is likely very rare in
Objective-C, except for one special case:

We do plan to special-case this for NSDictionary, which has keys of type
'id' while NSMutableDictionary's setter uses 'id <NSCopying>'.

Swift SVN r8638