[Local GC] Forbid inclusion of gcscan.h from VM and DAC directories (dotnet/coreclr...

[Local GC] Forbid inclusion of gcscan.h from VM and DAC directories (dotnet/coreclr#10332)

* Forbid inclusion of gcscan.h from VM and DAC directories

* Address code review feedback - hoist IsGCHeapInitialized check to an assert, since the heap should definitely be initialized if we are validating objects that ostensibly came from it

Commit migrated from https://github.com/dotnet/coreclr/commit/d5906ff3e0d0f864c1432f1f39d586ca3d79fbb6