swift234

IRGen: Fudge passing large ObjC structs as byvals.

Push LLVM attribute generation from expandAbstractCC into getFunctionSignature and CallEmission so that they can generate sret and/or byval attributes per-argument according to the calling convention. Copy our bogus rule for emitting sret returns (more than three elements in the explosion) and reuse it to pass large struct values as byvals rather than as explosions. This should be good enough to get both 'NSRect' and
'NSRange', 'NSSize' etc. to pass correctly to ObjC methods. Next step is to set the AbstractCC correctly for imported func decls so that standalone C functions follow the same bogus rule.

Swift SVN r3993