[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-sles-build-only` running on `rocm-worker-hw-04-sles` while building `clang,llvm` at step 5 "compile-openmp".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/140/builds/15775

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

```
Step 5 (compile-openmp) failure: build (failure)
...
9.893 [3602/32/3457] Building arm_sme_streaming_attrs.inc...
9.898 [3601/32/3458] Building arm_mve_builtin_aliases.inc...
9.904 [3600/32/3459] Building arm_mve_builtins.inc...
9.907 [3599/32/3460] Linking CXX static library lib/libLLVMX86Disassembler.a
9.911 [3598/32/3461] Building arm_sve_streaming_attrs.inc...
9.913 [3597/32/3462] Building AttrSubMatchRulesParserStringSwitches.inc...
9.917 [3596/32/3463] Building DiagnosticSerializationKinds.inc...
9.920 [3595/32/3464] Building LinalgStructuredOps.cpp.inc...
9.924 [3594/32/3465] Building AttrParserStringSwitches.inc...
9.925 [3593/32/3466] Building Options.inc...
FAILED: tools/clang/include/clang/Driver/Options.inc 
cd /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build && /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/bin/llvm-tblgen -gen-opt-parser-defs -I /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/clang/include/clang/Driver -I/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/clang/include -I/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/tools/clang/include -I/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.build/include -I/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include /home/botworker/bbot/builds/openmp-offload-sles-build/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/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
9.927 [3593/31/3467] Building AttrSpellingListIndex.inc...
9.928 [3593/30/3468] Building LinalgStructuredOps.h.inc...
9.929 [3593/29/3469] Linking CXX static library lib/libLLVMX86Desc.a
9.931 [3593/28/3470] Building AttrParsedAttrKinds.inc...
9.934 [3593/27/3471] Building AttrTemplateInstantiate.inc...
9.938 [3593/26/3472] Building AttrPCHWrite.inc...
9.942 [3593/25/3473] Building AttrPCHRead.inc...
9.943 [3593/24/3474] Building TestOps.cpp.inc...
9.947 [3593/23/3475] Building TestOps.h.inc...
9.947 [3593/22/3476] Building AttrParsedAttrImpl.inc...
9.994 [3593/21/3477] Building arm_sve_builtins.inc...
10.390 [3593/20/3478] Building CXX object lib/Object/CMakeFiles/LLVMObject.dir/IRSymtab.cpp.o
11.706 [3593/19/3479] Building AMDGPUGenCallingConv.inc...
12.497 [3593/18/3480] Building AMDGPUGenMCPseudoLowering.inc...
12.603 [3593/17/3481] Building CXX object lib/MC/MCParser/CMakeFiles/LLVMMCParser.dir/AsmParser.cpp.o
12.824 [3593/16/3482] Building AMDGPUGenPostLegalizeGICombiner.inc...
12.862 [3593/15/3483] Building AMDGPUGenDisassemblerTables.inc...
13.030 [3593/14/3484] Building AMDGPUGenPreLegalizeGICombiner.inc...
13.074 [3593/13/3485] Building AMDGPUGenMCCodeEmitter.inc...
13.159 [3593/12/3486] Building AMDGPUGenRegBankGICombiner.inc...
14.134 [3593/11/3487] Building AMDGPUGenSubtargetInfo.inc...
14.269 [3593/10/3488] Building AMDGPUGenSearchableTables.inc...
16.697 [3593/9/3489] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
In file included from /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include/llvm/CodeGen/MachineStableHash.h:17:0,
                 from /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include/llvm/CodeGen/MachineOutliner.h:21,
                 from /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include/llvm/CodeGen/TargetInstrInfo.h:28,
                 from /home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp:59:
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include/llvm/ADT/StableHashing.h: In function ‘llvm::StringRef llvm::get_stable_name(llvm::StringRef)’:
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include/llvm/ADT/StableHashing.h:57:15: warning: unused variable ‘P0’ [-Wunused-variable]
   auto [P0, S0] = Name.rsplit(".content.");
               ^
/home/botworker/bbot/builds/openmp-offload-sles-build/llvm.src/llvm/include/llvm/ADT/StableHashing.h:62:15: warning: unused variable ‘S1’ [-Wunused-variable]
   auto [P1, S1] = Name.rsplit(".llvm.");
               ^

```

</details>

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