Explicitly call BindExpression in GetSpeculativeSymbolInfo, instead of Bind, to be sure that we don't return type expression info instead of actual expression info.

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

Use the SnapshotManager to store the variable types as it was already doing this, and handle nested scopes correctly.

Add GetDeclaredSymbol support for nullability in variable declarations. Not handled are linq or foreach iteration variable declarations.

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

Fixes #29819

Fixes #30067

Address minor PR comments.

Remove feature flag requirement from IDE quick info tests.
Adjust assert for attributes semantic model.

Make nullable semantic analysis depend on whether the user is actually consuming the nullable feature in any way.

* Correctly report nullability of tuples in the semantic model:
- Store the original tuple literal rather than just its type
- Visit the original literal without diagnostics only in the nullability walker to understand its type
- Update the generator to allow force visiting fields in the nullable re-writer
- Update type checker to allow checking of converted types too
- Update various tuple tests to be correct
- Add extra tests

Plumb speculative model through to analyzers. Expand the tests of the speculative model to show both inputs to the nullable walker and inferences from the nullable walker in the speculated code.

* Update implicity declared type expression nullability:
- When updating the implicit type in a declaration also update the bound type declaration nullability
- Record nullability for inferred foreach variable
- Add nullable public api tests
- Add nullable tests

Refactors out ApplyConversion into a more aptly named VisitConversion. Adjusts the parameters to take the node being converted, and optionally the original boundconversion. When the boundconversion is passed it, the calculated nullability is now tracked through nested levels of conversions between the top-level boundconversion and the underlying converted node. Removed TrackInferredTypesThroughConversions, as this is now being handled by VisitConversion.

This PR additionally fixes a conversion issue where user-defined conversions that added nullability were not then marking the result type of that conversion as nullable.

This makes 2 changes:
1. Reinfer the GetEnumerator method based on nullable analysis of the
foreach expression type.
2. Use that information to update the collection element type based on
that same analysis.

Disable assertion that we are now hitting on enum initializers and file https://github.com/dotnet/roslyn/issues/34919 to follow up.