tests/libcxx: We're using the old libc++ test format which will be removed

[StephanTLavavej]

Running tests/libcxx emits a warning that we're using the old libc++ test format. libc++'s maintainers are planning to remove that: https://lists.llvm.org/pipermail/libcxx-dev/2020-June/000885.html

Curtis J Bezault (@cbezault) says:

We should just provide our own lit.cfg file for libcxx which can mirror the ones we already have for std and tr1.

[StephanTLavavej]

The emitted warning is:

S:\GitHub\STL\out\build\x86>python tests\utils\stl-lit\stl-lit.py ..\..\..\llvm-project\libcxx\test\std\containers\sequences\vector\vector.data -o testing_x86.log
stl-lit.py: S:/GitHub/STL/llvm-project/libcxx/test/lit.cfg:49: note: Using configuration variant: stl
stl-lit.py: S:\GitHub\STL\tests\utils\stl\test\target_info.py:43: note: The locale Czech_Czech Republic.1250 is not supported by your platform. Some tests will be unsupported.
stl-lit.py: S:/GitHub/STL/llvm-project/libcxx/test/lit.cfg:59: note: Using the old libc++ testing format
[...]