diagnostics: complain about undeclared string tokens

String literals, which allow for better error messages, are (too)
liberally accepted by Bison, which might result in silent errors.  For
instance

    %type <exVal> cond "condition"

does not define “condition” as a string alias to 'cond' (nonterminal
symbols do not have string aliases).  It is rather equivalent to

    %nterm <exVal> cond
    %token <exVal> "condition"

i.e., it gives the type 'exVal' to the "condition" token, which was
clearly not the intention.

Introduce -Wdangling-alias to catch this.

* src/complain.h, src/complain.c: Add support for -Wdangling-alias.
(argmatch_warning_args): Sort.
* src/symtab.c (symbol_check_defined): Complain about dangling
aliases.
* doc/bison.texi: Document it.
* tests/input.at (Dangling aliases): New test.

src/complain.h

@@ -47,6 +47,7 @@ typedef enum
   {
     warning_conflicts_rr,
     warning_conflicts_sr,
+    warning_dangling_alias,
     warning_deprecated,
     warning_empty_rule,
     warning_midrule_values,
@@ -104,6 +105,7 @@ typedef enum
 
     Wconflicts_rr     = 1 << warning_conflicts_rr,
     Wconflicts_sr     = 1 << warning_conflicts_sr,
+    Wdangling_alias   = 1 << warning_dangling_alias,
     Wdeprecated       = 1 << warning_deprecated,
     Wempty_rule       = 1 << warning_empty_rule,
     Wmidrule_values   = 1 << warning_midrule_values,
@@ -118,7 +120,7 @@ typedef enum
 
     /**< All above warnings.  */
     Weverything       = ~complaint & ~fatal & ~silent,
-    Wall              = Weverything & ~Wyacc
+    Wall              = Weverything & ~Wdangling_alias & ~Wyacc
   } warnings;
 
 /** Whether the warnings of \a flags are all unset.