Previously the `CMAKE_GENERATOR_INSTANCE` value was used only to filter
the instances reported by the Visual Studio Installer tool. If the
specified install location is not known to the VS Installer, but the
user provided a `version=` field, check for the installation directly
on disk.
Fixes: #21639, #22197
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
