[clang] [sanitizer] Fix empty string in unsupported argument error for -fsanitize-trap (PR #136549)

[Thurston Dang via cfe-commits]

thurstond wrote:

> > Perhaps a better fix would be to correct the toString function
> 
> I agree, but adding groups to `toString` breaks its other uses. What do you think about separate `toStringWithGroups`?

Hmm, that's a good point. I like your `toStringWithGroups` idea!

https://github.com/llvm/llvm-project/pull/136549