[llvm] [MC] Do not crash on switching to sections of unknown types. (PR #92380)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

Most commits don't append a trailing `.` to the title.

The description can be made to say that this fixes a crash for MCAsmStreamer. MCObjectStreamer already accepts unknown types.

https://github.com/llvm/llvm-project/pull/92380