backend: revamp the handling of symbol types

Currently it is the front end that passes the symbol types to the
backend.  For instance:

  %token <ival> NUM
  %type <ival> exp1 exp2
  exp1: NUM { $$ = $1; }
  exp2: NUM { $<ival>$ = $<ival>1; }

In both cases, $$ and $1 are passed to the backend as having type
'ival' resulting in code like `val.ival`.  This is troublesome in the
case of api.value.type=union, since in that the case the code this:

  %define api.value.type union
  %token <int> NUM
  %type <int> exp1 exp2
  exp1: NUM { $$ = $1; }
  exp2: NUM { $<int>$ = $<int>1; }

because in this case, since the backend does not know the symbol being
processed, it is forced to generate casts in both cases: *(int*)(&val)`.
This is unfortunate in the first case (exp1) where there is no reason
at all to use a cast instead of `val.NUM` and `val.exp1`.

So instead delegate the computation of the actual value type to the
backend: pass $<ival>$ as `symbol-number, ival` and $$ as
`symbol-number, MULL`, instead of passing `ival` before.

* src/scan-code.l (handle_action_dollar): Find the symbol the action
is about, not just its tyye.  Pass both symbol-number, and explicit
type tag ($<tag>n when there is one) to b4_lhs_value and b4_rhs_value.

* data/bison.m4 (b4_symbol_action): adjust to the new signature to
b4_dollar_pushdef.

* data/c-like.m4 (_b4_dollar_dollar, b4_dollar_pushdef): Accept the
symbol-number as new argument.

* data/c.m4 (b4_symbol_value): Accept the symbol-number as new
argument, and use it.
(b4_symbol_value_union): Accept the symbol-number as new
argument, and use it to prefer ready a union member rather than
casting the union.
* data/yacc.c (b4_lhs_value, b4_rhs_value): Accept the new
symbol-number argument.
Adjust uses of b4_dollar_pushdef.
* data/glr.c (b4_lhs_value, b4_rhs_value): Adjust.

* data/lalr1.cc (b4_symbol_value_template, b4_lhs_value): Adjust
to the new symbol-number argument.
* data/variant.hh (b4_symbol_value, b4_symbol_value_template): Accept
the new symbol-number argument.

* data/java.m4 (b4_symbol_value, b4_rhs_data): New.
(b4_rhs_value): Use them.
* data/lalr1.java: Adjust to b4_dollar_pushdef, and use b4_rhs_data.

data/java.m4

@@ -219,21 +219,56 @@ m4_define([b4_position_type], [b4_percent_define_get([[api.position.type]])])
 ## ----------------- ##
 
 
+# b4_symbol_value(VAL, [SYMBOL-NUM], [TYPE-TAG])
+# ----------------------------------------------
+# Expansion of $$, $1, $<TYPE-TAG>3, etc.
+#
+# The semantic value from a given VAL.
+#
+# VAL: some semantic value storage (typically a union).
+#   e.g., yylval
+# SYMBOL-NUM: the symbol number from which we extract the
+#   type tag.
+# TYPE-TAG, the user forced the <TYPE-TAG>.
+#
+# The result can be used safely, it is put in parens to avoid nasty
+# precedence issues.
+m4_define([b4_symbol_value],
+[m4_ifval([$3],
+          [(($3)($1))],
+          [m4_ifval([$2],
+                    [b4_symbol_if([$2], [has_type],
+                                  [((b4_symbol([$2], [type]))($1))],
+                                  [$1])],
+                    [$1])])])
+
+
 # b4_lhs_value([TYPE])
 # --------------------
 # Expansion of $<TYPE>$.
 m4_define([b4_lhs_value], [yyval])
 
 
-# b4_rhs_value(RULE-LENGTH, NUM, [TYPE])
-# --------------------------------------
-# Expansion of $<TYPE>NUM, where the current rule has RULE-LENGTH
+# b4_rhs_data(RULE-LENGTH, POS)
+# -----------------------------
+# Expansion of $<TYPE>POS, where the current rule has RULE-LENGTH
+# symbols on RHS.
+#
+# In this simple implementation, %token and %type have class names
+# between the angle brackets.
+m4_define([b4_rhs_data],
+[yystack.valueAt ($1-($2))])
+
+# b4_rhs_value(RULE-LENGTH, POS, SYMBOL-NUM, [TYPE])
+# --------------------------------------------------
+# Expansion of $<TYPE>POS, where the current rule has RULE-LENGTH
 # symbols on RHS.
 #
 # In this simple implementation, %token and %type have class names
 # between the angle brackets.
 m4_define([b4_rhs_value],
-[(m4_ifval($3, [($3)])[](yystack.valueAt ($1-($2))))])
+[b4_symbol_value([b4_rhs_data([$1], [$2])], [$3], [$4])])
+
 
 # b4_lhs_location()
 # -----------------