VS: Allow CMAKE_GENERATOR_INSTANCE to specify portable instance

Brad Kingrequested to merge
brad.king/cmake:vs-instance into master

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

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