[Attributor] Cleanup `::initialize` of various AAs

This commit cleans up the ::initialize method of various AAs in the
following ways:
  - 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.