Update the documentation and CMake file for Visual Studio generators.

By default, CMake uses a 32-bit toolchain, even when on a 64-bit platform targeting a 64-bit build. However, due to the size of the binaries involved, this can cause linker instabilities (such as the linker running out of memory). Guide people to the correct solution to get CMake to use the native toolchain.

llvm-svn: 303912