Move FunctionType conversion into CGCall.cpp:

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@56047 91177308-0d34-0410-b5e6-96231b3b80d8