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

Fixes #37310
Fixes #37279

* Add IsDeclaredReadOnly and IsEffectivelyReadOnly to public API

* Fix build. Add simple SemanticModel test.

* Fix Roslyn users of IMethodSymbol. Expand semantic model test.

* Fix visibility of MethodSymbol.vb members

* Implement ReadOnly method API in CodeGenerationConstructedMethodSymbol

* Cleanup usings and change NotImplementedException to ExceptionUtilities.Unreachable

* Simplify down to IMethodSymbol.IsReadOnly

* Fix IsReadOnly API for reduced extension methods. Test ref readonly ref
properties.

* Ensure that non-reduced extension methods are never 'readonly'

* Update doc

* Reduced extension methods are not declared readonly, they're effectively readonly.

* Remove unused copy of extension methods test source

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.

* IDE Changes

* tests

* PR comments

NOTE: binding of `ref readonly` parameters is NYI

Introducing new `RefKind.In` - to represent readonly ref parameters.
Also introduces _very preliminary_  ReturnsByReadonlyRef API to represent readonly ref returns.

Internal APIs are aligned with these public APIs.
VB Symbol Display for ByRef returns is adjusted according to the latest design decision.

Add docs for ISymbol and its derived interfaces that say that they should not be implemented outside Roslyn

These were accidentally changed to UTF-16LE during a recent scripted change.  This went unnoticed because the encoding change didn't register in CodeFlow (indeed if you view a shelve of this change in CodeFlow it will report no changes).  UTF-16 does not work well with Git as it is often interpreted as a binary file.  Moving back to UTF-8 so our GitHub experience works well for customers.

closes #66 (changeset 1403787)

Background:

As discussed in the dev team all hands all code under the GitHub dotnet foundation is using a single process for contribution, API review, infrastructure and coding style. The idea is to present a unified view to our customer and give them a single story for contributing to any project under the dotnet foundation.

https://github.com/dotnet/corefx/wiki/Contributing#c-coding-style

The coding style transition is automated using a Roslyn based rewrite tool:

https://github.com/dotnet/codeformatter

This will be applied in stages across our developer tree. Right now the focus is on the Open directory as this is what is being presented on github. Code owners will be contacted before the transition happens.

Note: this is a soft style requirement. There are no build errors that come from this change.
 (changeset 1403394)

Splits Microsoft.CodeAnalysis, Microsoft.CodeAnalysis.CSharp and Microsoft.CodeAnalysis.VisualBasic into portable and desktop assemblies.

Existing Core\Source, CSharp\Source and VisualBasic\Source directories are renamed to Core\Portable, CSharp\Portable and VisualBasic\Portable.
New sibling Desktop folders are added and non-portable source is moved there.
"System.Runtime" references has to be removed in order for the portable project system magic to automatically add facade references.
 (changeset 1281686)

We need to be more conscious about producing synthesized symbols during lowering in order to enable EnC of method bodies whose lowering produce such symbols (e.g. lambdas, iterators, dynamic sites, etc.).

     This change refactors synthesized symbols in C# so that all symbols produced during lowering phase implement an interface (ISynthesizedMethodBodyImplementationSymbol) that can be used in the emit phase to retrieve information necessary for updating the corresponding metadata entities should they enter EnC.

     I also attempted to somewhat improve and increase consistency of the naming of synthesized symbols involved in this change.

     No new functionality is added by this change. (changeset 1247789)

Public API Change: Rename IFieldSymbol.AssociatedPropertyOrEvent , IMethodSymbol.AssociatedPropertyOrEvent and INamedTypeSymbol. AssociatedEvent APIs to AssociatedSymbol to accommodate for future language changes. (changeset 1210026)