improve C ABI

I borrow some ideas from clang's ABIInfo.h and TargetInfo.cpp.
LLVMType is replaced with ArgType, which is similar to clang's ABIArgInfo,
and I also merge attrs of FnType into it.

Now ABI implementation doesn't need to insert hidden return pointer
to arg_tys of FnType. Instead it is handled in foreign.rs.

This change also fixes LLVM assertion failure when compiling MIPS target.