Tests that FAIL in C++17 mode

[Jonathan Wakely]

On 11/10/16 11:49 +0100, Jonathan Wakely wrote:
>The following tests are failing with -std=gnu++17
>
>20_util/shared_ptr/cons/constexpr.cc
>20_util/unique_ptr/cons/constexpr.cc

I suspect these two fail because of guaranteed copy-elision in C++17
mode, so they should use { target c++14_down }

>23_containers/unordered_set/allocator/ext_ptr.cc
>29_atomics/atomic_integral/operators/bitwise_neg.cc
>ext/profile/mutex_extensions_neg.cc
>
>I haven't looked into why yet.
>
>If anyone wants to investigate they can put that list of files into
>$objdir/$target/libstdc++-v3/testsuite/testsuite_files and then run
>make check RUNTESTFLAGS=--target_board=unix/-std=gnu++17
>