To make it clearer this is about whether the library supports the debug
mode at all, not whether the debug mode is enabled. Per comment by Nico
Weber on IRC.
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
Louis Dionne