[Attributor] Guarantee getAAFor not to update AA in the manifestation stage

If we query an AA with `Attributor::getAAFor` in `AbstractAttribute::manifest`, the AA may be updated.
This patch makes use of the phase flag in Attributor, and handle `getAAFor` behavior according to the flag.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D86635