[llvm] [BPF] Handle data-carrying enums (PR #155783)

[via llvm-commits]

eddyz87 wrote:

Let's change the name to: `Support for DW_TAG_variant_part in BTF generation`. Data carrying enum is not an official term anyway (is this how algebraic data types are called these days?).

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