swift234

SILGen: Better handle function conversions with both a signature and representation change

We need to be careful to do them in the right order when there's
bridging involved.

This still doesn't completely solve the problem, because of blocks
appearing as function arguments. For example, Sema right now allows
a conversion from @convention(block) (@convention(block) () -> ()) -> ()
to (() -> ()) -> (), but the thunk runs into difficulties because the
bridging thunk only converts the outermost function, and the
re-abstraction thunk hits an assertion on the inner thunk because it
is not able to perform representation changes.

One solution would be to actually unify bridging thunks with
re-abstraction thunks, now that re-abstraction thunks can convert
between AST types.

Progress on <rdar://problem/22470076>.

Swift SVN r31883