[libcxx] Respect CMAKE_MSVC_RUNTIME_LIBRARY wrt whether to use the debug CRT

If CMAKE_MSVC_RUNTIME_LIBRARY isn't set, then CMake defaults to
the debug CRT, if CMAKE_BUILD_TYPE is set to Debug. If
CMAKE_MSVC_RUNTIME_LIBRARY is set though, that overrides any
implicit defaults.

Match this in libcxx's own manual linking logic. This allows
decoupling the debug CRT from the CMAKE_BUILD_TYPE and allows users
to configure their builds exactly how they want.

Differential Revision: https://reviews.llvm.org/D155561