* data/lalr1.cc (yyreduce): Compute the resulting state before
running the user action so that yylhs is a valid symbol.
(yyerrorlab): Since yylhs is complete (it knows its type), we can
simply call yy_destroy_ to destroy $$ on YYERROR invocations.
* tests/c++.at (AT_CHECK_VARIANTS): Test YYERROR with variants.