[lld] [ELF] Add isStaticRelSecType to simplify SHT_REL/SHT_RELA testing. NFC (PR #85893)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

LGTM too. 

I assume that if there was an `isDynamicRelSecType` then `SHT_REL` and `SHT_RELA` would also be included in that, but also the other compressed dynamic formats too.

https://github.com/llvm/llvm-project/pull/85893