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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `amdgpu-offload-ubuntu-22-cmake-build-only` running on `rocm-docker-ubu-22` while building `clang,llvm` at step 4 "annotate".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/203/builds/1137

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 4 (annotate) failure: '../llvm-zorg/zorg/buildbot/builders/annotated/amdgpu-offload-cmake.py --jobs=32' (failure)
...
-- Performing Test HAVE_POSIX_REGEX -- success
-- Performing Test HAVE_STEADY_CLOCK -- success
-- Performing Test HAVE_PTHREAD_AFFINITY -- success
-- Configuring done
-- Generating done
-- Build files have been written to: /home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build
@@@BUILD_STEP build cmake config@@@
Running: ninja
[1/517] Building CXX object tools/llvm-config/CMakeFiles/llvm-config.dir/llvm-config.cpp.o
[2/517] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build/tools/clang/include/clang/Driver/Options.inc 
cd /home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build && /home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build/bin/llvm-tblgen -gen-opt-parser-defs -I /home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/llvm-project/clang/include/clang/Driver -I/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/llvm-project/clang/include -I/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build/tools/clang/include -I/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build/include -I/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/llvm-project/llvm/include /home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/llvm-project/clang/include/clang/Driver/Options.td --write-if-changed -o tools/clang/include/clang/Driver/Options.inc -d tools/clang/include/clang/Driver/Options.inc.d
/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
[3/517] Generating VCSRevision.h
[4/517] Linking CXX executable bin/llvm-config
ninja: build stopped: subcommand failed.
['ninja'] exited with return code 1.
The build step threw an exception...
Traceback (most recent call last):
  File "/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build/../llvm-zorg/zorg/buildbot/builders/annotated/amdgpu-offload-cmake.py", line 38, in step
    yield
  File "/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build/../llvm-zorg/zorg/buildbot/builders/annotated/amdgpu-offload-cmake.py", line 29, in main
    run_command(["ninja"])
  File "/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/build/../llvm-zorg/zorg/buildbot/builders/annotated/amdgpu-offload-cmake.py", line 51, in run_command
    util.report_run_cmd(cmd, cwd=directory)
  File "/home/botworker/bbot/amdgpu-offload-ubuntu-22-cmake-build-only/llvm-zorg/zorg/buildbot/builders/annotated/util.py", line 49, in report_run_cmd
    subprocess.check_call(cmd, shell=shell, *args, **kwargs)
  File "/usr/lib/python3.10/subprocess.py", line 369, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['ninja']' returned non-zero exit status 1.
@@@STEP_FAILURE@@@

```

</details>

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