GitHub: Onboard libcxx tests

[StephanTLavavej]

We use libcxx's test suite to validate MSVC's STL (with both the MSVC and Clang/LLVM compilers). We need to migrate this test infrastructure to our GitHub CI process. (We use libcxx's test sources, but not their test harness, due to Windows/Posix differences.)

[StephanTLavavej]

git has been adding support for partial clones, which may be useful for the libcxx subset of the llvm monorepo: https://git-scm.com/docs/partial-clone

[StephanTLavavej]

After looking into it further, it appears that partial clone support is still a work in progress, and can't really be used yet.