VS: Suppress VS 16.9+ default /permissive flag

Brad Kingrequested to merge
brad.king/cmake:vs-16.9-ConformanceMode into master

VS 16.9 changed its ConformanceMode setting from a BoolProperty to an EnumProperty, and set it to true by default. This causes the /permissive flag to be passed to the compiler even if not requested by the project. In this case, add an explicit ConformanceMode default setting to suppress the flag unless the project or user passes it.

Fixes: #21945 (closed)
Backport: release

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