VS: Allow CMAKE_GENERATOR_INSTANCE to specify portable instance
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 (closed), #22197 (closed)
Edited by Brad King