Try to replace/simplify `copy_fn_decl` by leveraging `copy_fndecl_with_name`

[JAChapmanII]

When generating the DECL_PRE_FN/DECL_POST_FN we copy the guarded function decl since they share many properties. See if it's possible to shorten or delete the copy_fn_decl function with the older copy_fndecl_with_name used to generate copies for ctdtors and operators.

[asutton]

copy_fnecl_with_name hands the function off to middle end. We haven't generatated code for the function at the point we call copy_fn_decl.