(yyparse): New local var yylerrsp, to record the
top of the location stack's error locations.
(yyerrlab): Set it. When discarding a token, push its location
onto yylerrsp so that we don't lose track of the error's end.
(yyerrlab1): Now is only the target of YYERROR, so that we can
properly record the location of the action that failed. For GCC
2.93 and later, insert an __attribute__ ((__unused__)) to avoid
GCC warning about yyerrlab1 being unused if YYERROR is unused.
(yyerrlab2): New label, which yyerrlab now falls through to.
Compute the error's location by applying YYLLOC_DEFAULT to
the locations of all the symbols that went into the error.