[BOLT][NFC] Rename DataAggregator::BranchInfo to TakenBranchInfo

Align the name to its counterpart `FTInfo` which avoids name aliasing
with llvm::bolt::BranchInfo and allows to drop namespace specifier.

Test Plan: NFC

Reviewers: maksfb, rafaelauler, ayermolo, dcci

Reviewed By: dcci

Pull Request: https://github.com/llvm/llvm-project/pull/92017