Since LLVM/Windows emulates Visual Studio, CMake sets
CMAKE_C_SIMULATE_ID="MSVC" but does not set MSVC. Thus, our build
system needs to enable most (but not all) of the Visual Studio features
when CMAKE_C_SIMLUATE_ID="MSVC". Support for LLVM/Windows is currently
undocumented because it isn't a standalone build environment. (It
requires the Visual Studio and Windows SDK headers and link libraries.)
Closes#786
7f200a2acc