need function decl/type in get_parm_info

[DJ Delorie]

I'm working on converting PROMOTE_PROTOTYPES to be a target hook,
sensitive to any attributes that might have been added to the function
decl.  Unfortunately, one of the places it's used - get_parm_info() in
c-decl.c - happens before current_function_decl is defined, and there
doesn't appear to be an obvious way to get the function type or decl.

Alternately, can the promotions be deferred until later, when the decl
is available?  If so, where's the best place to move it to?