[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 `openmp-offload-amdgpu-runtime` running on `omp-vega20-0` while building `clang,llvm` at step 5 "compile-openmp".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/30/builds/14785

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

```
Step 5 (compile-openmp) failure: build (failure)
...
6.499 [3452/32/891] Building X86GenAsmWriter1.inc...
6.552 [3451/32/892] Building X86GenInstrMapping.inc...
6.557 [3450/32/893] Building Options.inc...
6.562 [3449/32/894] Building Options.inc...
6.574 [3448/32/895] Building Options.inc...
6.591 [3447/32/896] Building Options.inc...
6.611 [3446/32/897] Building Options.inc...
6.620 [3445/32/898] Building Options.inc...
6.629 [3444/32/899] Building Options.inc...
6.691 [3443/32/900] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc /home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.build/tools/clang/include/clang/Driver/Options.inc 
cd /home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.build && /home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.build/bin/llvm-tblgen -gen-opt-parser-defs -I /home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.src/clang/include/clang/Driver -I/home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.src/clang/include -I/home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.build/tools/clang/include -I/home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.build/include -I/home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.src/llvm/include /home/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.src/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/ompworker/bbot/openmp-offload-amdgpu-runtime/llvm.src/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
6.761 [3443/31/901] Building R600GenDAGISel.inc...
6.780 [3443/30/902] Building RISCVTargetParserDef.inc...
6.962 [3443/29/903] Building R600GenDFAPacketizer.inc...
7.048 [3443/28/904] Building X86GenAsmMatcher.inc...
7.097 [3443/27/905] Building R600GenMCCodeEmitter.inc...
7.328 [3443/26/906] Building R600GenRegisterInfo.inc...
7.407 [3443/25/907] Building R600GenSubtargetInfo.inc...
7.499 [3443/24/908] Building R600GenInstrInfo.inc...
7.606 [3443/23/909] Building X86GenDisassemblerTables.inc...
7.734 [3443/22/910] Building X86GenFoldTables.inc...
9.357 [3443/21/911] Building X86GenGlobalISel.inc...
9.793 [3443/20/912] Building X86GenFastISel.inc...
11.105 [3443/19/913] Building X86GenSubtargetInfo.inc...
11.538 [3443/18/914] Building X86GenDAGISel.inc...
14.187 [3443/17/915] Building X86GenInstrInfo.inc...
19.110 [3443/16/916] Building AMDGPUGenMCPseudoLowering.inc...
19.992 [3443/15/917] Building AMDGPUGenPostLegalizeGICombiner.inc...
20.120 [3443/14/918] Building AMDGPUGenPreLegalizeGICombiner.inc...
20.592 [3443/13/919] Building AMDGPUGenCallingConv.inc...
21.733 [3443/12/920] Building AMDGPUGenMCCodeEmitter.inc...
21.801 [3443/11/921] Building AMDGPUGenDisassemblerTables.inc...
21.949 [3443/10/922] Building AMDGPUGenSearchableTables.inc...
22.902 [3443/9/923] Building AMDGPUGenRegBankGICombiner.inc...
24.047 [3443/8/924] Building AMDGPUGenSubtargetInfo.inc...
29.677 [3443/7/925] Building AMDGPUGenGlobalISel.inc...
29.895 [3443/6/926] Building AMDGPUGenAsmMatcher.inc...
30.104 [3443/5/927] Building AMDGPUGenDAGISel.inc...
30.451 [3443/4/928] Building AMDGPUGenAsmWriter.inc...
31.415 [3443/3/929] Building AMDGPUGenInstrInfo.inc...
33.870 [3443/2/930] Building AMDGPUGenRegisterInfo.inc...
35.292 [3443/1/931] Building AMDGPUGenRegisterBank.inc...
ninja: build stopped: subcommand failed.

```

</details>

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