Allow specification of semantic predicates.

These changes allow users to prefix an action with %? to indicate that it
is a semantic predicate---an expression that is evaluated immediately (not
deferred, even in GLR nondeterministic mode) and causes a syntax error if
false.  In GLR parsers, this has the effect of killing one of a set of
split-off parses, just as would an ordinary syntax error.

Changelog:

    * NEWS: Describe new semantic-predicate feature.
    * data/c.m4 (b4_predicate_case): New definition.
    * data/java.m4 (b4_predicate_case): New definition.
    * data/glr.c (yyimmediate): Add definition.
    (yydoAction): Remove comment, now obsolete.
    Do YY_REDUCE_PRINT here.
    (yyglrReduce): Alter comment to indicate that semantic values
    need not be deferred.
    Remove YY_REDUCE_PRINT from here; done in yydoAction.
    (yyprocessOneStack): Pass immediate flag.
    Delete stacks rejected by predicates in newly split-off parsers.
    Change handling of yyerr so that only current stack gets deleted
    when semantic predicate fails.
    (yyfillin): Don't crash if a semantic value is unresolved (as may
    happen in predicate rules).
    Copy lr state as well in debugging mode.
    Update comment on setting of yysval to include yyloc as well.
    (yy_reduce_print): Add yynormal argument.  Perform fillin properly.
    Report unresolved RHS values.
    (yyimmediate): New table.
    * src/gram.h (struct rule): Add is_predicate field.
    * src/output.c (user_actions_output): Use b4_predicate_case for
    predicates.
    (prepare_symbols): Output yyimmediate.
    * src/scan-gram.l: Add %? token, SC_PREDICATE state.
    * src/scan-code.l (code_props_rule_action_init): Add is_predicate
    argument.
    * src/scan-code.h (struct code_props): Add is_predicate field.
    (code_props_rule_action_init): New interface.
    * src/parse-gram.y (%?{...}): New token.
    (rhs): Add %?{...} rule.
    * src/parse-gram.c: Regenerate.
    * src/parse-gram.h: Regenerate.
    * src/reader.c (grammar_current_rule_action_append): Add
    immediate argument.
    (grammar_midrule_action): Use new interface for
    code_props_rule_action_init.
    (grammar_current_rule_action_append): Ditto.
    (packgram): Transfer is_predicate value.
    * src/reader.h (grammar_current_rule_action_append): New interface.
    * doc/bison.texinfo: Document semantic predicates (%?).

    * data/glr.c (yylhsNonterm, yyisDefaultedState,yyDefaultAction)
    (yygetLRActions,yynewGLRStackItem,yyaddDeferredAction,yyinitStateSet)
    (yyinitGLRStack,yyexpandGLRStack,yyupdateSplit,yymarkStackDeleted)
    (yyundeleteLastStack,yyglrShift,yyglrShiftDefer,yydoAction,yyglrReduce)
    (yyidenticalOptions,yymergeOptionSets,yyresolveStates,yyresolveAction)
    (yyresolveLocations,yyresolveValue,yyreducePrint): Update parameter
    names in comments and mention all parameters.
    (struct yyGLRState): Fix description of yyposn field.
    (yyresolveLocations): Correct comment so as not to imply action when
    yyn1==0.

NEWS

@@ -51,6 +51,15 @@ Bison News
   use of the %error-verbose directive is deprecated in favor of
   %define parse.error "verbose".
 
+** Semantic predicates
+
+  The new, experimental, semantic-predicate feature allows actions of
+  the form %?{ BOOLEAN-EXPRESSION }, which cause syntax errors (as for
+  YYERROR) if the expression evaluates to 0, and are evaluated immediately
+  in GLR parsers, rather than being deferred.  The result is that they
+  allow the programmer to prune possible parses based on the values of
+  runtime expressions.
+
 * Changes in version 2.5 (????-??-??):
 
 ** Named References Support