Add contains method to associative containers. This patch implements P0458R2, adding contains to map, multimap, unordered_map, unordered_multimap, set, multiset, unordered_set, and unordered_multiset.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366170 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed