* Test compositions

* Eliminate TestExportProvider

* Fixes and caching

* Debugger proxies

* Scopes

* Feedback and more fixes

* RemoteHostOptions

* Throw without timeout on invalid thread switches

* EventCollectorTests requires a main thread

This reverts commit cbb5cf10.

* 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

Co-authored-by: NCyrusNajmabadi <cyrus.najmabadi@gmail.com>
Co-authored-by: NDavid Wengier <dawengie@microsoft.com>

Fixes #46075

Fixes #46047

#45765 enhance this analyzer to support detecting unnecessary inline SuppressMessageAttribute suppressions. This led to a regression when processing idToPragmasMap for unhandled IDs, which is fixed by this change.

by aggregating overloads, also reduce the number of items returned

* InteractiveHost for .NET Core

* Merge fix

* Fixes

* Typo

* Workaround for https://github.com/dotnet/interactive-window/issues/156

* Fix

* Update OptProf config

* Fix deployment of rsp files

* Fix signing.

* Clean up unit tests dependencies.

* Remove unnecessary references.

* Fix reset interactive command

* Fix OptProf config

* Add missing project refs

* Fix InteractiveHost deployment

* Process rsp file fully in InteractiveHost process and send back results.

* Fix build from VS.

* Retrieve metadata resolver parameters from InteractiveHost process.

Simplify keeping search paths in-sync between InteractiveHost and InteractiveEvaluator.

* Fixes

* Fixes

* Check for failures to create process in unit tests

* Test

* Pending buffers

* Fix submission project creation

* Logging

* Serialize exceution and process initialization

* Fix reference completion, resolution.

* MetadataReferenceResolver fixes

* Fix failing .NET Core tests

* Target netcoreapp.

* Set DOTNET_ROOT

* Fix NGEN/OptProf

* Avoid loading Microsoft.CodeAnalysis.Scripting

* Fix

* Feedback

* Feedback

* Feedback

* Comment on dependent target

* Disable interpreting of strings as DateTime during deserialization:

* Merge fix

* Use TestHost consistently

* Comments

* Fix

* RenameTestHost

* Renaming

Fixes #45757

Fixes #45661
* Add test from contributor at #45742
Fixes #43834

This change standardizes the reference assemblies we use for compilation in our unit test to be the actual .NET Framework reference assemblies. 

Prior to this change they were a mix of reference and implementation assemblies from non-RTM versions of the .NET Framework. In order to map the existing assemblies to their respective target framework version I worked with the servicing team and we mapped them to the closest possible TFM based on their file versions. The bulk of the change is essentially mechanically moving from names like `mscorlib.v4_30319_17929` to `Net451.mscorlib`.

Unfortunately in several cases the non-RTM assemblies didn't cleanly map to a RTM TFM. This was particularly problematic when migrating the `v4_30319` references as this is a mix of a `net40` and `net451` versions. This caused issues in our code base because these references were freely mixed with actual `net40` and `net451` references in the code base. Most of the time this went unnoticed because the test didn't expose the gaps in the APIs. In a lot of cases though, particularly when mixing a version of `System.Core` that did or didn't have `ExtensionsAttribute`, this required some manual inspection on my part and a fix. In pretty much every case this was a straight forward fix but it did mean the change was less mechanical than I would have preferred.

To make this change easier to review I've broken it up into three commits:

1. The actual changes to `Roslyn.Test.Utilities` that move us to the new references.
1. The changes to our test code which required more than a simple straight forward reference update or explicitly involved retargetting code. These are changes I think deserve stronger scrutiny than the more mechanical aspects of this PR.
1. The mechanical changes to move from the old references to the new ones. This is the bulk of the change but is for the most part renames. 

The benefits of this change are the following:

1. Have our test more closely mirror customer scenarios by using official reference assemblies.
1. Improve the readability of the tests by referring to assemblies by their target framework, which is familiar to most developers, instead of their file version, which is familiar to servicing team only.
1. Reduces the build output size of Roslyn by **~5GB**

The internal PR is here https://github.com/dotnet/roslyn-internal/pull/1954

Note: I will be making more changes in this area in future PRs. But I wanted to keep the mechanical portion of this change into its own PR. Further PRs will be smaller but less mechanical. 

Fixes #44178

Builds on top of #44848, which added support to detect unnecessary pragma suppressions.

Fixes #45446