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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> 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.

Yes. `isDynamicRelSecType` would include SHT_RELR and SHT_AARCH64_AUTH_RELR SHT_ANDROID_RELA, but I don't find a call site where the function would be useful.

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