Fixes #30198

Merge master-vs-deps back into master (no 16.8 changes in master-vs-deps)

XAML LSP Support

Merge release/dev16.8-preview1 to master

Merge Parameter Hints to Master

Report teh project name that we were unable to recover a symbol from.

Merge master to master-vs-deps

Fixes #45761

* Support nullable annotations on unconstrained type parameters

Support `?` annotations on unconstrained type parameter references:
- `?` annotation is allowed for type parameters that are not constrained to reference types or value types
- `?` annotation is an error when used on unconstrained type parameter with `/langversion:8`

Support `where T : default` constraint:
- `default` constraint is allowed for method type parameters in overridden or explicitly implemented methods to disambiguate between overloads defined in the base or interface
- `default` constraint is an error with `/langversion:8`
- `default` constraint is an error when used outside of method overrides or explicit implementations
- `default` constraint is an error when the type parameter is constrained

Fixes https://github.com/dotnet/roslyn/issues/29146

Fix #46157

Merge release/dev16.8-preview1-vs-deps to master-vs-deps

Merge release/dev16.8-preview1 to master

Merge pull request #46233 from dotnet/merges/release/dev16.8-preview1-to-release/dev16.8-preview1-vs-deps

Merge release/dev16.8-preview1 to release/dev16.8-preview1-vs-deps

Replacing ILanguageServerProtocol with AbstractRequestHandlerProvider.
Refactoring common Extensions method code into generic helpers.

* Use explicit namespace for proprietary resources

The Roslyn code base originated with all of our resource reference /
impl DLLs being checked into a single utility project. At the point we
moved to Git and went OSS these single utility project was factored into
two projects for a number of reasons.

1. Keeping DLLs we didn't have license for private
1. Letting us put large binaries into a NuPkg file vs. polluting Git
   history

When this split occured the namespaces for the utility projects were not
updated. That means many types in our public and private resource
projects have the same name. This makes it **really** difficult to
determine where types and resources are coming from.

This change adds an explicit namespace to the proprietary resource
assembly. This will help make it clear which resources come from it.

* Fix version

* Remove version ambigutiy

* Fix test failure

Rename types to not collide with the much more relevant Workspace equivalents

* Move to the .NET 5 SDK

This is necessary for us to test features like covariant returns because
we need to be able to target `net5.0` in our unit test projects.

Issues encountered in this update:

- WinRT is no longer supported in .NET 5. As a result we no longer multi-target
.NET Core in our WinRT test assembly as well as having to disabling a
few WinRT tests in others.  https://github.com/dotnet/runtime/issues/37672
- Change our Debugger Proxy tests to react to a runtime change
https://github.com/dotnet/runtime/commit/c362923cae8543b5092b2115f169411ee7dd4763
- Work around .NET5 P6 bug https://github.com/dotnet/sdk/issues/11862

Remove wait for out of process start from OnLoadedAsync as it has not…

Merge master to master-vs-deps

E.g.
`[installDir]\Common7\IDE\CommonExtensions\Microsoft\ManagedLanguages\VBCSharp\LanguageServices\InteractiveHost\Desktop\InteractiveHost64.exe`

Enable BasicLineCommit.CommitOnSave

Enable CSharpInteractiveCommands.VerifyPreviousAndNextHistory

Switch 'use auto prop' to use NRT and make us more null safe.

Fix InteractiveHost32 build

Single overload - Inline Parameter Name Hints

Fixes #46148

Remove wait for out of process start from OnLoadedAsync as it has not cancellation token.  The wait will instead occur in ActivateAsync with cancellation