[clang] [CIR] Lower member-pointer casts in constant emitter (PR #198910)

[via cfe-commits]

adams381 wrote:

Incremental diff for review (cast commits only, on top of #197555):

https://github.com/llvm/llvm-project/compare/adams381:cir-data-member-offset-nfc...adams381:cir-data-member-cast-stacked

GitHub shows the full branch vs `main` in the PR diff because the base branch lives on the fork. Use the compare link above for the stacked view.

https://github.com/llvm/llvm-project/pull/198910