how to get the library DECL for a built-in function

[Martin Sebor]

I'm looking for a way to get the FUNCTION_DECL for the library
(i.e., non-built-in) form of a function given the corresponding
built-in DECL.  Is there an API I can all with either the built
-in DECL or its code to get it in the middle end?

In C, what I'm looking for appears to be DECL_CHAIN(decl), at
least for the example I looked at.  I.e., the library libdecl
for a corresponding __bultin_xxx DECL is DECL_CHAIN (DECL).

But in C++, it looks like it's the other way around and it's
the built-in decl that's the DECL_CHAIN(DECL) for the built-in
DECL, and AFAIK there's no good way to get from the latter to
the former.

Thanks
Martin