Visual Studio 2017 supports multiple instances installed on a single
machine. We use the Visual Studio Installer tool to enumerate instances
and select one. Once we select an instance for a given build tree, save
the result in `CMAKE_GENERATOR_INSTANCE` so we can re-configure the tree
with the same instance on future re-runs of CMake.
Fixes: #17268
Add cache entry `CMAKE_GENERATOR_INSTANCE` to hold the instance location
persistently across re-runs of CMake in a given build tree.
For now we reject the option by default if explicitly set. It will be
implemented on a per-generator basis. Pass the setting into try_compile
project generation. Add a RunCMake.GeneratorInstance test to cover
basic use cases for the option. Verify that `CMAKE_GENERATOR_INSTANCE`
is empty by default, and that it is rejected when the generator does not
support a user setting.
Issue: #17268
