[BOLT][NFC] Refactor BAT metadata data structures

Hide the implementations of `FuncHashes` and `BBHashMap` classes,
getting rid of `at` accessors that could throw an exception.

Test Plan: NFC

Reviewers: ayermolo, maksfb, dcci, rafaelauler

Reviewed By: rafaelauler

Pull Request: https://github.com/llvm/llvm-project/pull/86353