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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

I've fixed the build failure with https://github.com/llvm/llvm-project/commit/8578b816fa9050c33561f06d631459e12645953a.

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