Visual Studio provides toolchains that are themselves built for 32-bit
or 64-bit host architectures. By default it uses the 32-bit tools, but
it can be told to prefer the 64-bit tools on 64-bit hosts. Extend the
`CMAKE_GENERATOR_TOOLSET` specification to provide a way to request
use of the 64-bit host tools.
Closes: #15622
Now that we know the year component of this VS version we
can add it to the generator name. For convenience, map
the name without the year to the name with the year.