[libc++] Add a CMake option to control whether the debug mode is supported

Some libc++ builds may want to disable support for the debug mode,
for example to reduce code size or because the current implementation
of the debug mode requires a global map. This commit adds the
LIBCXX_ENABLE_DEBUG_MODE CMake option and ties it into the test
suite.

It also adds a CI job to test this configuration going forward.

Differential Revision: https://reviews.llvm.org/D88923