swift234

Select correct IR types for direct arguments to C/@objc functions.

Reapply r13532 fixes for dealing with arguments that should be exploded
for C/Obj-C functions.

This gets us a bit closer to properly generating the correct types for
arguments. The remaining piece is generating all the argument types at
once rather than one at a time, which also requires being able to always
generate Clang types for the Swift types we see in the signatures of
these functions.

Swift SVN r13638