Move FunctionType conversion into CGCall.cpp:

 - Added CodeGenTypes::GetFunctionType, taking a CGFunctionInfo.
 - Updated Obj-C runtimes to use this instead of rolling the
   llvm::FunctionType by hand.
 - Killed CodeGenTypes::{ConvertReturnType, DecodeArgumentTypes}.

Add ABIArgInfo class to encapsulate ABI decision of how to lower types
to LLVM.
 - Will move to target sometime soon.

llvm-svn: 56047