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

[Nick Desaulniers via llvm-commits]

================
@@ -4352,19 +4352,11 @@ def stack_usage_file : Separate<["-"], "stack-usage-file">,
 def fextend_variable_liveness_EQ : Joined<["-"], "fextend-variable-liveness=">,
   Group<f_Group>, Visibility<[ClangOption, CC1Option]>,
   HelpText<"Extend the liveness of user variables through optimizations to "
-           "prevent stale or optimized-out variable values when debugging. Can "
-           "be applied to all user variables, or just to the C++ 'this' ptr. "
-           "May choose not to extend the liveness of some variables, such as "
-           "non-scalars larger than 4 unsigned ints, or variables in any "
-           "inlined functions.">,
+           "prevent stale or optimized-out variable values when debugging."
----------------
nickdesaulniers wrote:

```
/home/llvm-libc-buildbot/buildbot-worker/libc-x86_64-debian/libc-x86_64-debian-dbg-bootstrap-build/llvm-project/clang/include/clang/Driver/Options.td:4356:3: error: Expected comma before next argument
  Values<"all,this,none">,
  ^
```

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