This respects the CMAKE_MSVC_RUNTIME_LIBRARY option for selecting
the right CRT to use.
Add a CI configuration that tests building this way.
Based on a patch by Andrew Ng.
The test config files end up accumulating and duplicating a fair
bit of cmake-specific logic here; if preferred, we could also add
that in `libcxx/test/CMakeLists.txt` and export a few more variables
to `cmake-bridge.cfg.in` instead.
Differential Revision: https://reviews.llvm.org/D155560
e777e44546