Move System.Convert code to shared partition (dotnet/coreclr#10178)

* Move System.Convert code to shared partition
* Use ReferenceEquals for type equality

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: alexperovich

src/Common/src/CoreLib/System.Private.CoreLib.Shared.projitems

@@ -86,6 +86,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\IEnumerator.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\IEqualityComparer.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Collections\IList.cs"/>
+    <Compile Include="$(MSBuildThisFileDirectory)System\Convert.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Action.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\ApplicationException.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\ArgumentNullException.cs"/>