VS: Add support for per-config sources
Since !701 (merged) the VS generators have known how to generate per-config sources. We've now converted most other code paths to support per-config sources, so drop the check that disallows it.
This leaves only per-config support for precompiled headers and unity build transformations, but those are optional features that can be addressed later.
Fixes: #18233 (closed)
Issue: #19789 (closed)
Edited by Brad King