First half of support for N3657; heterogenous lookups for set/multiset

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@188241 91177308-0d34-0410-b5e6-96231b3b80d8
More support for N3657; tests for is_transparent

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@188242 91177308-0d34-0410-b5e6-96231b3b80d8