[llvm] [BPF] Support for `DW_TAG_variant_part` in BTF generation (PR #155783)

[via llvm-commits]

eddyz87 wrote:

For the reference: I checked BTF generation for kernel BPF selftests and there are (unsurprisingly) no differences with and w/o this patch. Should be ready to land once the remaining nits are addressed.

https://github.com/llvm/llvm-project/pull/155783