Abseil associative containers having keys of a string type provide
so-called heterogeneous lookup: they allow either `std::string`,
`std::string_view`, or `absl::Cord` values in lookup.

On the other hand, using either of associative containers should not
require compiling the cord library unless the use of `absl::Cord` is
explicitly requested.

The solution is to forward-declare `absl::Cord` and a few functions just
to support heterogeneous lookup, make the associative containers use
these functions only, and require users of `absl::Cord` to link with
the cord library explicitly.

fixes abseil#1868