yysyntax_error: adjust prior fixes for branch-2.5's lalr1.cc.

yysyntax_error: adjust prior fixes for branch-2.5's lalr1.cc.

On master, there is no yychar in lalr1.cc, but there is on
branch-2.5, and the prior cherry-pick of "Fix handling of yychar
manipulation in user semantic actions" wasn't adjusted for that
difference.
* data/lalr1.cc (yy::parser::parse): Translate yychar before
every use of yytoken, and add comments explaining this approach.
* tests/conflicts.at (%error-verbose and consistent errors):
Extend to test yychar manipulation with lalr1.cc.