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

[Fangrui Song via llvm-commits]

MaskRay wrote:

In the test you added
```
  // Test that switching to a group section with no associated signature name
  // doesn't crash.
  C.Streamer->switchSection(
      C.Ctx->getELFSection("foo", ELF::SHT_PROGBITS, ELF::SHF_GROUP));
```

I am still not convinced we should take the change. Perhaps we should disallow SHF_GROUP without a signature in switchSection. This isn't valid in ELF.

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