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/language.h

@@ -778,10 +778,6 @@ extern enum language set_language (enum language);
 
 extern void range_error (const char *, ...) ATTRIBUTE_PRINTF (1, 2);
 
-/* Data:  Does this value represent "truth" to the current language?  */
-
-extern int value_true (struct value *);
-
 /* Misc:  The string representing a particular enum language.  */
 
 extern enum language language_enum (const char *str);