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