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