[MC][ELF] Support for zero flag section groups

This change introduces support for zero flag ELF section groups to LLVM.
LLVM already supports COMDAT sections, which in ELF are a special type
of ELF section groups. These are generally useful to enable linker GC
where you want a group of sections to always travel together, that is to
be either retained or discarded as a whole, but without the COMDAT
semantics. Other ELF assemblers already support zero flag ELF section
groups and this change helps us reach feature parity.

Differential Revision: https://reviews.llvm.org/D95851

GitOrigin-RevId: 16af97393346ad636298605930a8b503a55eb40a
17 files changed