Move AbstractCC into SILType and make it an attribute of SILTypes for functions. Add a ConvertCCInst to represent calling convention conversions. Give SILFunctions a linkage attribute. Add logic to SILGen to calculate these attributes for SILConstants based on their attached decls.
IRGen doesn't use these new attributes yet. I'll hook that up when I move mangling over.
Swift SVN r4886
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
