Merge 'features/NullableReferenceTypes' into master

Remove "Make readonly" suggestion from fixed size buffers

Command handler to replace `@$"` with `$@"` when typing interpolated verbatim string

Mono doesn't support all of the security options used on in the
compiler server on the the desktop .NET framework, but it does
support PipeOptions.CurrentUserOnly, which provides the same
security guarantees.

Add code fix to add 'Shadows' modifier

Split the 'move declaration near reference' feature into a feature and *service*

Split the 'move declaration near reference' feature into a feature and *service*.  That way other features can leverage the service portion.

The project needed an upgrade to netcoreapp2.1 and I deleted
an obsolete workaround by upgrading benchmark.net.

Fixes #30722

Merge dev16.0.x to master

Merge dev15.9.x to dev16.0.x

* Add test for tuple operator increment

* Ignore tuple names when comparing return types to parameter types in increment/decrement operators

* Add test for tuple identity conversion operator

* Ignore tuple names when comparing conversion operator parameter and return types

* Add test for tuple conversion to base type

* Ignore tuple names when determining whether parameter or return type contains a conversion operator

* Add test for tuple binary operator

* Filter out type conflict warnings

Propagate values from AbstractProject to VisualStudioProjectCreationInfo

* Invert the order of binding for InitializerExpressions and CreationExpressions:
- Push intializer binding into the respective creation expression bindings so that initializer binding takes place *after* the creation, ensuring any referenced out vars have already been inferred, preventing infinite loops
- Add tests to show behavior

Merge dev16.0.x to master

* Use Arcade dotnet install scripts

* Update roslyn-tools build tasks to 1.0.0-beta3.18523.2

* DotNet version check, remove LocateDotNet.

Fix naming consistency of ConvertAutoPropertyToFullProperty refactoring

* Ensure that the compilers produce double.NaN values in IEEE canonical form.
Fixes #30439

Merge dev16.0.x to master

Remove stale comment.

Correct 15.9 publish data

Contributing page: reference Unix build instructions

Delete 0

EnsureNonNullTypesAttributeExists not called for NoPIA