no $$ in %destructor = no unset value warnings?

[Joel E. Denny]

a. value is likely set/constructed = $$ appears in semantic action
b. value is likely destroyed = $$ appears in %destructor

That is, I think Bison shouldn't warn about !a when !b.  However, Bison 
currently considers a value to be likely destroyed by a %destructor 
regardless of whether $$ appears in the %destructor.  I think that's 
inconsistent.  Moreover, it might be convenient to override a per-type or 
default %destructor by defining an empty per-symbol %destructor, which 
would suppress the warnings:

  %destructor { free ($$); }
  %destructor { } symbol_with_no_value

What do you think?

Joel