Merge dev16.0.x to dev16.0-preview2

Fix gold-bar when an analyzer starts up, but not all references are restored/found

Remove NonNullTypes context and other unnecessary information stored in TypeSymbolWithAnnotations. (#30913)

Remove NonNullTypes context and other unnecessary information stored in TypeSymbolWithAnnotations.

Fixes #30845.

Explicitly handle nullability analysis for unconstraint type parameters.

Related to #29981, #29993

Add some additional shims to VisualStudioProjectTracker

Implement new interfaces for adding files

Merge dev16.0.x to dev16.0-preview2

Fixes https://github.com/dotnet/roslyn/issues/30966
Implements the spec change described in https://github.com/dotnet/roslyn/pull/30670
See also https://github.com/dotnet/roslyn/issues/30989

Merge pull request #31049 from jasonmalinowski/fix-project-reference-conversion-during-project-unload

Make sure we convert project references back on project removal

Fix Edit and Continue in CPS projects

Add bunch of exclusions to dead code analysis for special methods

The problem was our CPSProject type no longer was implementing the ENC
interfaces the project system expects; this copy/pastes the same code
over that's being used for AbstractLegacyProject.

If we don't do this, projects will end up with dangling project
references, which isn't something we want. Even if the project later
reloads, the fact there's still old ProjectIds floating around causes
us to fail to convert to P2P references again later.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/714766.

Merge dev16.0.x to dev16.0-preview2

Adding new training data and updating training scenarios