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

[Sinkevich Artem via cfe-commits]

ArtSin 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`?

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