Auto-generate the reference assemblies and separate type forwards (dotnet/corefx...

Auto-generate the reference assemblies and separate type forwards (dotnet/corefx#35557)

* Auto-generate the reference assemblies

* Add back type forwards.

* Revert "Add back type forwards."

This reverts commit dotnet/corefx@3acf74aabb2e3ba90ac7280d541723d13d72c74a.

* Add back TypeForwardedTo in .Forward.cs files

* Add new type forward files to the csproj files

* Add back if-defs and move TestUtilities comment from ref to source

* Remove duplicate definitions, delete unnecessary manual files,

* Fix build breaks

* Add back missing AsyncMethodBuilderAttr to S.Runtime ref

* Fix typo between XmlDocument and XPath.XDocument

* Add type forward fild missed from previous commit

* Fix ECDiffieHellmanCngPublicKey internal ctor for netfx 4.6.1

* Remove "this type was moved to comments" from the type forwards.

* Add back the Xml and Debugger* attributes to Xml.ReaderWriter

* Add back struct layout attributes

* Add back attributes missing (highlighted from api compat)

* Add missing toolbox item attribute in s.data.common.

* Add missing struct layout attributes and remove unnecessary additions

* Add back InterfaceTypeAttribute

Commit migrated from https://github.com/dotnet/corefx/commit/16689aa0a933517dc45465fc6edb6223222414a6