[Remarks] BitstreamRemarkParser: Refactor error handling (#156511)

In preparation of larger changes to the bitstream remark format,
refactor the error handling code in the BitstreamRemarkParser.

Main change: move the various static helper methods into the parser
helper classes, so we don't need to pass around as many args. Calling
`error(...)` inside the helper classes now automatically prepends the
current block being parsed to the error message.

NFCI (except for error messages on invalid bitstream files).

Pull Request: llvm/llvm-project#156511

Author: tobias-stadler