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

[Bill Wendling via llvm-commits]

bwendling wrote:

> > Could you add a test?
> 
> Perhaps a unittest in llvm/unittests/Support/CommandLineTest.cpp

Sure. I'll do that soon.

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