[Attributor] Cleanup ::initialize of various AAs
This commit cleans up the ::initialize method of various AAs in the
- If an associated function is required, give up on declarations. This was discovered as a real problem when lots of llvm.dbg.XXX call sites were assumed noreturn until proven otherwise. That does not make any sense and caused huge regressions and missed deductions.
- Require more associated declarations for function interface AAs.
- Use the IRAttribute::initialize to determine if function interface AAs can be used in IPO, don't replicate the checks (especially isFunctionIPOAmendable) all over the place. Arguably the function declaration check should be moved to some central place to.