Merge NullableAnnotation.NotApplicable and Disabled to None, and rename NullableFlowState.NotApplicable to None for consistency.

Use NullableAnnotation.NotAnnotated always for value type annotations in the public API.

Previously, the annotation was NotAnnotated if the type reference was from source and Disabled if the type reference was from metadata.

Fixes #34412

Remove support for explicit `object` generic type constraints

Add initial support for 'notnull' generic type constraint.

See https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-05-15.md
Also fixes #36005.

Closes #28834.
Closes #30061.

Implement an alternative way to break cycles while calculating IsValueType/IsReferenceType for a type parameter. (#35145)

Fixes #30081.

Treat unconstrained type parameters declared in `#nullable disable` context as having an oblivious nullability in case they are substituted with a reference type. (#34797)

As apposed to possibly nullable or not nullable reference type.
Fixes #29980.
Fixes #34844.

Update the stub API to remove Nullability and replace with NullableAnnotation and NullableFlowState, and update the
the rewriter to use that instead of the Nullability enum.

- Rename `TypeSymbolWithAnnotations` to `TypeWithAnnotations`
- Rename its type field from `TypeSymbol` to `Type`
- Fields of Symbols that are of type `TypeWithAnnotations` that are currently named `Type` would be renamed `TypeWithAnnotations`
Fixes #33736

* Eliminate unspeakable NullableAnnotation values.
* Dynamic invocations result in an oblivious state
Fixes #33638
* Fix up expected diagnostics
Due to #33924
* Rename NullableAnnotation.Unknown to NullableAnnotation.Oblivious

* Add ITypeSymbol.IsReadOnly public API

* Fix build failures in Roslyn.sln

* Update doc comment

The work for generic `unmanaged struct` constraints ended up threading
through the correct `Compilation` object in many of the cases where I
was using `Compilation` to calculate whether or not nullability needed
to be checked for constraints. Used that whenever possible.

* Add ITypeSymbol.IsUnmanagedType and ITypeSymbol.IsRefLikeType

Fixes #30427 and #30426

Closes #30171.
Closes #29838.

 Adjust the way nullable annotations are represented in metadata.

Closes #30075.
Closes #30065.
Closes #29594.
Closes #29683
Closes #29662

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

Also implemented detection of nullability mismatch in constraints for an implicit interface implementation.