These aren't really orthogonal concerns--you'll never have a @thick @cc(objc_method), or an @objc_block @cc(witness_method)--and we have gross decision trees all over the codebase that try to hopscotch between the subset of combinations that make sense. Stop the madness by eliminating AbstractCC and folding its states into SILFunctionTypeRepresentation. This cleans up a ton of code across the compiler.
I couldn't quite eliminate AbstractCC's information from AST function types, since SIL type lowering transiently created AnyFunctionTypes with AbstractCCs set, even though these never occur at the source level. To accommodate type lowering, allow AnyFunctionType::ExtInfo to carry a SILFunctionTypeRepresentation, and arrange for the overlapping representations to share raw values.
In order to avoid disturbing test output, AST and SILFunctionTypes are still printed and parsed using the existing @thin/@thick/@objc_block and @cc() attributes, which is kind of gross, but lets me stage in the real source-breaking change separately.
Swift SVN r27095
The set of attributes that make sense at the AST level is increasingly divergent from those at the SIL level, so it doesn't really make sense for these to be the same. It'll also help prevent us from accidental unwanted propagation of attributes from the AST to SIL, which has caused bugs in the past. For staging purposes, start off with SILFunctionType's versions exactly the same as the FunctionType versions, which necessitates some ugly glue code but minimizes the potential disruption.
Swift SVN r27022
