[ELF] Simplify SectionBase::partition handling and make it live by default. NFC

Previously an InputSectionBase is dead (`partition==0`) by default.
SyntheticSection calls markLive and BssSection overrides that with markDead.

It is more natural to make InputSectionBase live by default and let
--gc-sections mark InputSectionBase dead.

When linking a Release build of clang:

* --no-gc-sections:, the removed `inputSections` loop decreases markLive time from 4ms to 1ms.
* --gc-sections: the extra `inputSections` loop increases markLive time from 0.181296s to 0.188526s.
  This is as of we lose the removing one `inputSections` loop optimization (4374824c).
  I believe the loss can be mitigated if we refactor markLive.