[flang] use TBAAForest in TBAABuilder

This is important to ensure that tags end up in the same trees that were
created in the FIR TBAA pass. If they are in different trees then
everything in one tree will be assumed to MayAlias with everything in the
other tree. This leads to poor performance.

@vzakhari requested that the old (not-per-function) trees are
maintained so I left the old test intact.

PR: https://github.com/llvm/llvm-project/pull/68437