swift234

Annotate thunks with the [thunk] attribute during SILGen.

Swift generates two entry points to @objc methods where one of
them is a thunk, and the inliner happily inlines the swift code
into the @objc thunk, effectively doubling the code size of some
@objc classes.

The performance inliner already knows not to inline large functions
into callers that are marked as thunks. This commit adds the [thunk]
attribute to the @objc thunks in an attempt to reduce code size.

rdar://22403108

Swift SVN r31498