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

[Ivan Kosarev via llvm-commits]

kosarev 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

Having the assert without the loading code erroring out signature-less group sections means we'll just crash on loading such ELFs.

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