[BOLT][NFC] Expose YAMLProfileWriter::convert function

The function is to be used by YAML profile emission in BAT mode for
BinaryFunctions not covered by BAT tables (same as in original binary).

Test Plan: NFC

Reviewers: rafaelauler, ayermolo, dcci, maksfb

Reviewed By: dcci

Pull Request: https://github.com/llvm/llvm-project/pull/76909