[clang] [CIR] Combined virtual + non-virtual base offset (PR #192617)

[via cfe-commits]

xiongzile wrote:

@andykaylor 
This PR currently only handles virtual and non-virtual base offsets.
For test, I reuse the virtual-base-cast.cpp you mentioned.

virtual-base-cast.cpp's changes:
- remove compile option: -fno-clangir-call-conv-lowering
- ``cir.binop(add,`` updated to ``cir.add``


https://github.com/llvm/llvm-project/pull/192617