[AST] Make AliasSetTracker work on BatchAA

D138014 restricted AST to work on immutable IR. This means it is
also safe to use a single BatchAA instance for the entire AST
lifetime, instead of only batching parts of individual queries.

The primary motivation for this is not compile-time, but rather
having a central place to control cross-iteration AA, which will
be used by D137958.

Differential Revision: https://reviews.llvm.org/D137955