[lto] Add nth_parm_type_p, alloc_parm_types, vec_heap2parm_types.

[Kazu Hirata]

Hi Ian,

>>+tree *
>>+nth_parm_type_p (tree parmtypes, int n)
> 
> 
> By gcc convention a function which ends with "_p" is a predicate, not
> a function which returns a pointer.  Please use a different name.

A very good point, but then I am having trouble coming up with a good name.  How 
about nth_parm_typep (without '_')?  I got a hint from call_expr_argp.

Kazu Hirata