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

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

Thanks for the test. However, it is still not clear to me what the motivation for the change was. Maybe similar changes should be made to the specializations of the `cl::parser` class? Should the change be reflected in the [documentation](https://llvm.org/docs/CommandLine.html)?

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