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

[Alexander Richardson via llvm-commits]

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

Definitely easier to read, especially once there are more cases to check.

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