[DSLX] give specific error on conflict marker token

Tools like clang give a specific error when they detect a conflict marker, it'd be nice to do similar.

Start by locating the DSLX lexer or parser entry point that handles conflict-marker tokens and the related diagnostic tests. Compare the existing behavior with the requested clang-style diagnostic, then add coverage showing that a conflict marker produces a specific error.

Written by the indexing model from the issue text.