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

Fixes #35933

Adds a public API, `NullableContext GetNullableContext(int position)`, for determining the nullable context at a particular file location, and types supporting that API.

* Add all nullable-related warnings to ErrorFacts

* Fix test failures and add a bit more coverage

Fixes #36241

Fixes #37187

Implements https://github.com/dotnet/csharplang/issues/2389
Fixes #35958

Note that the switch expression conversion is not a standard implicit conversion.  Specifically, it should not be considered as a valid implicit conversion as input to a user-defined conversion, as that could permit two chained user-defined conversions.

Also changes binding of switch expression to ECMA so it can interact properly with switch expressions.
Fixes #14965

Resolves #33344

Relates to #29821
Fixes #34409

Fixes #29819

Fixes #37111

Fixes #30067

The semantic model often creates ReducedExtensionMethods for the public API,
but the creation of ReducedExtensionMethod does constraint checking, which
requires a compilation, which isn't always available. This change removes constraint
checking from all creation of ReducedExtensionMethod and adds asserts that we
always have a compilation during constraint checking.

Fixes #37021

Address minor PR comments.

* Don't update analyzed nullability for param conversions
* Fix test types

Related to #35619.

Conflicts:
	src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

* 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

enabled the nullable feature, either on the command line
or via a nullable enable directive.

Support nullable variance in interface and partial method implementation.

Closes #35227.