Move value_true to value.h

I noticed that value_true is declared in language.h and defined in
language.c.  However, as part of the value API, I think it would be
better in one of those files.  And, because it is very short, I
changed it to be an inline function in value.h.  I've also removed a
comment from the implementation, on the basis that it seems obsolete
-- if the change it suggests was needed, it probably would have been
done by now; and if it is needed in the future, odds are it would be
done differently anyway.

Finally, this patch also changes value_true and value_logical_not to
return a bool, and updates some uses.

gdb/ada-lang.c

@@ -11765,13 +11765,13 @@ re_set_exception (struct breakpoint *b)
    user specified a specific exception, we only want to cause a stop
    if the program thrown that exception.  */
 
-static int
+static bool
 should_stop_exception (const struct bp_location *bl)
 {
   struct ada_catchpoint *c = (struct ada_catchpoint *) bl->owner;
   const struct ada_catchpoint_location *ada_loc
     = (const struct ada_catchpoint_location *) bl;
-  int stop;
+  bool stop;
 
   struct internalvar *var = lookup_internalvar ("_ada_exception");
   if (c->m_kind == ada_catch_assert)
@@ -11799,16 +11799,16 @@ should_stop_exception (const struct bp_location *bl)
 
   /* With no specific exception, should always stop.  */
   if (c->excep_string.empty ())
-    return 1;
+    return true;
 
   if (ada_loc->excep_cond_expr == NULL)
     {
       /* We will have a NULL expression if back when we were creating
 	 the expressions, this location's had failed to parse.  */
-      return 1;
+      return true;
     }
 
-  stop = 1;
+  stop = true;
   try
     {
       struct value *mark;