[llvm-branch-commits] [libc] [libc] Add `IN6_IS_ADDR_V4COMPAT` (PR #172646)

[Michael Jones via llvm-branch-commits]

https://github.com/michaelrj-google approved this pull request.

LGTM, please add a short description of what this function is to the commit description

https://github.com/llvm/llvm-project/pull/172646