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

Fixes #36091

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 #37310
Fixes #37279

Fixes #35933

Fixes #33685

Fixes #33821 

Fixes https://github.com/dotnet/roslyn/issues/37308

* EnC support of static local functions

* code review feedback

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

* Fix EnC debug information emitted for patterns

Update calculation of syntax offset to account for a new case when a node (a switch expression) that is associated with a variable, closure or lambda may share start offset with other node of the same kind (`expr switch { … } switch { … }`). Use the offset of the `switch` keyword instead of the starting offset of the expression to disambiguate.

Assign ordinals to variables synthesized for storing pattern values across cases. This is required to support complex patterns since we can no longer rely on the type of these variables to be distinct. This will require follow up in the IDE to disallow updating/adding/reordering the case clauses of switch expression which there an active statement is present within the switch statement. If the cases are unmodified the compiler guarantees that the order in which the synthesized variables are generated remains the same, so we can map the variables using their ordinal.

Mark all variables synthesized during lowering of switch expression as short-lived. Their lifespan is limited to the switch expression, which does not include a sequence point.

Disallow editing methods that contain switch expression. This is necessary until bugs https://github.com/dotnet/roslyn/issues/37232, https://github.com/dotnet/roslyn/issues/37237 are fixed.

* Feedback

* Update tests

* Add all nullable-related warnings to ErrorFacts

* Fix test failures and add a bit more coverage

* Emit tests

* Report rude edits for unsupported DIM operations

* Allow EnC tests to specify TFMs to build against

* EnC: test against netcoreapp30

* Test fixes

* Fix merge

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

* Top-level types enumeration refactoring

* Simplify DefinitionMap

* Include namespaces in synthesized definition map

* Add SynthesizedNamespaceSymbol

* Embedded Attributes

* Test synthesized attributes

* Clean up EnC tests

* Clean up symbol matcher tests

* Ref readonly return type, in parameter symbol matching tests

* Enable using ref readonly returns, in parameters, ref structs during debugging

* More tests

* ReadOnly struct members

* Fix langauge version mismatch

* Merge fix ups

* Feedback, skip tests

Fixes #37111

Fixes #30067

* EnC LocalSlotMappingTests for C# 8.0 patterns
* PDBTest
* EditAndContinueTest

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