[clang] [llvm] [Clang] Cleanup docs and comments relating to -fextend-variable-liveness (PR #124767)

[Jeremy Morse via llvm-commits]

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

LGTM on the belief that `-fextend-this-ptr-liveness` hasn't been in the driver for very long at all -- if that isn't the case, we end up having to ask questions about supporting it.

https://github.com/llvm/llvm-project/pull/124767