[clang] [CIR] Add basic support for data member pointers (PR #170939)

[Sirui Mu via cfe-commits]

https://github.com/Lancern approved this pull request.

LGTM in general.

I think the TargetLowering stuff in this PR is fine. Most of TargetLowering code is just boilerplate, without it we have to further delay the LLVM lowering of data member pointers to later PRs.

https://github.com/llvm/llvm-project/pull/170939