[dataflow] Extract arena for Value/StorageLocation out of DataflowAnalysisContext

DataflowAnalysisContext has a few too many responsibilities, this narrows them.

It also allows the Arena to be shared with analysis steps, which need to create
Values, without exposing the whole DACtx API (flow conditions etc).
This means Environment no longer needs to proxy all these methods.
(For now it still does, because there are many callsites to update, and maybe
if we separate bool formulas from values we can avoid churning them twice)

In future, if we untangle the concepts of Values from boolean formulas/atoms,
Arena would also be responsible for creating formulas and managing atom IDs.

Differential Revision: https://reviews.llvm.org/D148554

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