swift234

Add '[thin]' attribute to function types.

Some SIL operations only make sense on no-context functions, such as creating a context for a function with "curry" or specializing. We're going to need this concept in Swift for interop with C functions too, so I'm adding it to the Swift type system. This patch only adds the attribute bit to function types without exposing the attribute to Swift syntax or providing any means in Swift to produce values of thin function type or to type-check them.

Swift SVN r4416