[libc++] [P0458] Add map::contains and set::contains for heterogenous lookup missed in a17b1aed.

Commit rGa17b1aed added `bool contains(const key_type& x) const;` methods to associative containers, but didn't add `template<class K> bool contains(const K& x) const;` for heterogenous lookup.

Reviewed By: #libc, Quuxplusone, ldionne

Differential Revision: https://reviews.llvm.org/D100369