[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 `clang-hip-vega20` running on `hip-vega20-0` while building `clang,llvm` at step 3 "annotate".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/123/builds/12607

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

```
Step 3 (annotate) failure: '../llvm-zorg/zorg/buildbot/builders/annotated/hip-build.sh --jobs=' (failure)
...
-- Performing Test HAVE_PTHREAD_AFFINITY -- success
-- Configuring done
-- Generating done
-- Build files have been written to: /home/botworker/bbot/clang-hip-vega20/botworker/clang-hip-vega20/llvm
+ build_step 'Building LLVM'
+ echo '@@@BUILD_STEP Building LLVM@@@'
+ ninja
@@@BUILD_STEP Building LLVM@@@
[1/325] Building CXX object tools/llvm-config/CMakeFiles/llvm-config.dir/llvm-config.cpp.o
[2/325] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /home/botworker/bbot/clang-hip-vega20/botworker/clang-hip-vega20/llvm/tools/clang/include/clang/Driver/Options.inc 
cd /home/botworker/bbot/clang-hip-vega20/botworker/clang-hip-vega20/llvm && /home/botworker/bbot/clang-hip-vega20/botworker/clang-hip-vega20/llvm/bin/llvm-tblgen -gen-opt-parser-defs -I /home/botworker/bbot/clang-hip-vega20/llvm-project/clang/include/clang/Driver -I/home/botworker/bbot/clang-hip-vega20/llvm-project/clang/include -I/home/botworker/bbot/clang-hip-vega20/botworker/clang-hip-vega20/llvm/tools/clang/include -I/home/botworker/bbot/clang-hip-vega20/botworker/clang-hip-vega20/llvm/include -I/home/botworker/bbot/clang-hip-vega20/llvm-project/llvm/include /home/botworker/bbot/clang-hip-vega20/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/clang-hip-vega20/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
[3/325] Generating VCSRevision.h
[4/325] Linking CXX executable bin/llvm-config
ninja: build stopped: subcommand failed.
Step 6 (Configure LLVM Build) failure: Configure LLVM Build (failure)
@@@BUILD_STEP Configure LLVM Build@@@
+ cd /home/botworker/bbot/clang-hip-vega20/botworker/clang-hip-vega20/llvm
+ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DCMAKE_VERBOSE_MAKEFILE=1 '-DLLVM_TARGETS_TO_BUILD=AMDGPU;X86' '-DLLVM_ENABLE_PROJECTS=clang;lld;clang-tools-extra' '-DLLVM_ENABLE_RUNTIMES=compiler-rt;libcxx;libcxxabi;libunwind' -DCLANG_DEFAULT_LINKER=lld -DLIBCXX_ENABLE_SHARED=OFF -DLIBCXX_ENABLE_STATIC=ON -DLIBCXX_INSTALL_LIBRARY=OFF -DLIBCXX_INSTALL_HEADERS=OFF -DLIBCXXABI_ENABLE_SHARED=OFF -DLIBCXXABI_ENABLE_STATIC=ON -DLIBCXXABI_INSTALL_STATIC_LIBRARY=OFF -DCMAKE_INSTALL_PREFIX=/home/botworker/bbot/clang-hip-vega20/botworker/clang-hip-vega20/install -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_Z3_SOLVER=OFF -DLLVM_ENABLE_ZLIB=ON '-DLLVM_LIT_ARGS=-v -vv' /home/botworker/bbot/clang-hip-vega20/llvm-project/llvm
-- bolt project is disabled
-- clang project is enabled
-- clang-tools-extra project is enabled
-- compiler-rt project is disabled
-- cross-project-tests project is disabled
-- libc project is disabled
-- libclc project is disabled
-- lld project is enabled
-- lldb project is disabled
-- mlir project is disabled
-- openmp project is disabled
-- polly project is disabled
-- pstl project is disabled
-- flang project is disabled
-- Could NOT find LibXml2 (missing: LIBXML2_LIBRARY LIBXML2_INCLUDE_DIR) 
-- Could NOT find LibEdit (missing: LibEdit_INCLUDE_DIRS LibEdit_LIBRARIES) 
-- LLVM host triple: x86_64-unknown-linux-gnu
-- Native target architecture is X86
-- Threads enabled.
-- Doxygen disabled.
-- Ninja version: 1.10.1
-- Could NOT find OCaml (missing: OCAMLFIND OCAML_VERSION OCAML_STDLIB_PATH) 
-- Could NOT find OCaml (missing: OCAMLFIND OCAML_VERSION OCAML_STDLIB_PATH) 
-- OCaml bindings disabled.
-- LLVM default target triple: x86_64-unknown-linux-gnu
-- Building with -fPIC
-- Targeting AMDGPU
-- Targeting X86
-- LLD version: 20.0.0
-- Clang version: 20.0.0git
-- Registering ExampleIRTransforms as a pass plugin (static build: OFF)
-- Registering Bye as a pass plugin (static build: OFF)
-- Failed to find LLVM FileCheck
-- Google Benchmark version: v0.0.0, normalized to 0.0.0
-- Performing Test HAVE_GNU_POSIX_REGEX -- failed to compile
-- 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/clang-hip-vega20/botworker/clang-hip-vega20/llvm
+ build_step 'Building LLVM'
+ echo '@@@BUILD_STEP Building LLVM@@@'
+ ninja

```

</details>

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