Tweak fmgrtab generation so that the F_XXX macros that give OIDs for

built-in procedures are named after the prosrc field of pg_proc (ie,
the actual C function name), not the proname field.  This did not use
to make a difference back when the two were always the same, but in the
presence of overloaded proname values we'd best try to use the C name
instead.  AFAICT this change affects no existing code, but it is
necessary to be able to get at some built-in functions that no macro
was being generated for before.