Add more missing overrides to refs (dotnet/corefx#29738)

Add more missing overrides to refs (dotnet/corefx#29738)

Ran /t:GenerateReferenceSource and undid irrelevant changes, though I kept a few files where the changes were trivial (e.g. whitespace) and it seemed better to just accept the change to stay in sync with GenerateReferenceSource.

In general I only added missing overrides on non-sealed classes, but when there were already overrides included on sealed classes, I added them back for consistency.

Commit migrated from https://github.com/dotnet/corefx/commit/10aa8277de2521fb1e6a5b42d053ad8cc947be81