[BFI] Add a debug check for unknown block queries.

Summary:
Add a debug check for frequency queries for unknown blocks (typically blocks
that are created after BFI is computed but their frequencies are not
communicated to BFI.)

This is useful for detecting and debugging missed BFI updates.

This is debug build only and disabled behind a flag.

Reviewers: davidxl

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73920