The latest version of ilasm produced by coreclr is now a fully
independent executable; it no longer depends on having a full runtime
laid down next to it.

This means we can vastly simplify how it is deployde in our
infrastructure. The package can now be included and have ilasm manually
copied out. This is similar to the approach that we take for our
diasymreader native dependencies.

closes #37582
related #25930, #25144

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

Fixes #37310
Fixes #37279

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

Fixes #29819

* Remove RoslynProjectType

* Remove ToolsVersion, namespace from Project tags

* Remove updating of MSBuildAllProjects

* Update BuildBoss

* Delete DeployTooolsetCompiler

* Update config

* Disable some tests on Mac. Only have a mac coreclr job.

* Move the DeeplyNestedGeneric bar way down for Mac

* Add work item attributes for disabled tests

Closes #32540.
Closes #33083.

Merge remote-tracking branch 'origin/features/editorconfig-in-compiler' into merges/master-to-features/editorconfig-in-compiler

* Add ITypeSymbol.IsUnmanagedType and ITypeSymbol.IsRefLikeType

Fixes #30427 and #30426

Standardize the conditional IOperation verification skip method, and fix a bug that caused it to skip in exactly the wrong way.

* Use Arcade testing targets

* Skip CodeStyle tests on Mono

* Include net472 in test target frameworks for Mono

* Remove incorrect file existence check.

* Update ADO integration test yaml

Moved VB to non Lazy and Delegate based tree construction. Fixed bugs in the C# implementation and updated test text for the OperationKind standardization.

Add a unit test for the failing scenario. Verified the assert fires prior to the fix for the added unit test and the unit test fails.

In netcoreapp2.1 the behavior of the file system handling of invalid
characters changed. This broke the expectations of a number of our
tests.

https://github.com/dotnet/docs/issues/4483

After working with the CoreClr team it turns out part of our CoreClr
flakiness on Linux is due to bugs fixed in netcoreapp2.1. Given that and
that our consumers are all on netcoreapp2.1 already I'm moving us to
2.1.

* Move xunit package references to Roslyn.Toolsets.Xunit.targets

* Remove dependency on Microsoft.NET.Test.Sdk

When conditionally skipping tests for CoreClr I mistakenly
unconditionally skipped them in my attempt to document the reason for
the skip. The use of the Skip property causes the test to be
unconditionally skipped, not documenting the reason.

In addition to fixing this I prevented this from happening in the
future. It's very rare for a `[ConditionalFact]` to actually want to use
Skip as it means unconditionally skip. This is generally only for short
term bug work arounds. Hence I changed `ConditionalFactAttribute` as
follows:

- Added a new Skip property that is obsolete and has no setter.
Completely prevents the code `[ConditionalFact(..., Skip = )`
- Added a Reason property documentation in the case the test is skipped.
- Added an AlwaysSkip property in the case you always want to skip.

* Add API bool Compilation.IsSymbolAccessibleWithin(
            ISymbol symbol,
            ISymbol within,
            ITypeSymbol throughType = null)
Also correct comments that incorrectly imply that inaccessible methods can be a member of a method group.
Fixes https://github.com/dotnet/roslyn/issues/24518
Fixes https://github.com/dotnet/roslyn/issues/23842