Adam Ciarciński authored Nov 30, 2017 and Brad King committed Nov 30, 2017

Some versions of clang 5 (with libc++) have a problem with
`unordered_map` under `-std=gnu++1z`:

    /usr/include/c++/__hash_table:1134:43: error: conflicting types for '__hash_table<_Tp, _Hash, _Equal, _Alloc>'

Include `unordered_map` in our test source so that we reject this
combination and fall back to an older C++ standard flag.

Fixes: #17526