[llvm] [CommandLine] Set the 'ValueStr' with the selected argument (PR #90816)

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

My understanding is that `ValueStr` is just an implementation detail to support the `cl::value_desc` modifier, as in
```
static cl::opt<std::string> OutputFilename("o", cl::desc("Output filename"), cl::value_desc("filename"));
```
which result in
```
> tool --help
  -o <filename>  - Output filename
```
It would not be correct to reuse the same member for another purpose.

https://github.com/llvm/llvm-project/pull/90816