[llvm] [MC][ELF] Fix printing group signature symbols. (PR #112543)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> > Perhaps we should disallow SHF_GROUP without a signature in switchSection. This isn't valid in ELF.
> 
> Should the ELF loading code do the check then?

The section creation code can have an assert

https://github.com/llvm/llvm-project/pull/112543