Reverts r28087. We're going back to the C++ interface for SIMD, and the changes in this patch are needless complication for that design.
Swift SVN r28384
The design we landed on for SIMD is to define the vector types as nested types of their element, e.g. Float.Vector4, Int32.Vector2, etc. Update the Clang importer and other mapping facilities to match.
Swift SVN r28087
When -enable-simd-import is active, if we encounter a vector type, try to load the SIMD Swift module, and if successful, map float, double, and int vectors to SIMD.{Float,Double,Int}N types if they exist.
Swift SVN r27367