D10669: libc++ tests for C++17 features may fail if compiled with -std=c++1y / c++0x

Hello Marshall,

I am getting compilation errors while compiling the libc++ tests.
Would you be able to take a look at this, please?

⚙ D10669 Part 1 of N4279 - "Improved insertion interface for unique-key maps" introduced support for several C++17 features and added tests for them.
The tests are protected with "UNSUPPORTED: c++03, c++11, c++14", which is fine but seems to be incomplete.

We configure libc++ with the "-DLIBCXX_ENABLE_CXX1Y=ON" option that results in -std=c++1y flag for the compiler. C++1y is not in the UNSUPPORTED list and the tests fail to compile.

As C++1y was generally used before C++14 Standard publishing and the Standard has already been published, does it make sense to rename LIBCXX_ENABLE_CXX1Y to LIBCXX_ENABLE_CXX14 and use -std=c++14 instead of -std=c++1y?

User can also specify C++ version from the lit command line or in his own lit config, so the UNSUPPORTED list must be extended with c++1y / c++0x anyway.

I can do all the necessary changes if you approve them.