Update dependencies from https://github.com/dotnet/roslyn build 20201220.2 (#50086)

[master] Update dependencies from dotnet/roslyn

eng/Version.Details.xml

@@ -7,9 +7,9 @@
       <Uri>https://github.com/dotnet/arcade</Uri>
       <Sha>7ee8c2b620e66b3762d7a5a688dee8238770c86a</Sha>
     </Dependency>
-    <Dependency Name="Microsoft.Net.Compilers.Toolset" Version="3.9.0-3.20611.16">
+    <Dependency Name="Microsoft.Net.Compilers.Toolset" Version="3.9.0-4.20620.2">
       <Uri>https://github.com/dotnet/roslyn</Uri>
-      <Sha>f27619580ad5b2bdc4043ad867bc145a48d21af6</Sha>
+      <Sha>fd47ba4ddaa6c202823eb9d7ae5721bff306dcda</Sha>
     </Dependency>
   </ToolsetDependencies>
 </Dependencies>

eng/Versions.props

@@ -23,7 +23,7 @@
     </MajorVersion>
     <MinorVersion>
     </MinorVersion>
-    <MicrosoftNetCompilersToolsetVersion>3.9.0-3.20611.16</MicrosoftNetCompilersToolsetVersion>
+    <MicrosoftNetCompilersToolsetVersion>3.9.0-4.20620.2</MicrosoftNetCompilersToolsetVersion>
   </PropertyGroup>
   <PropertyGroup>
     <!-- Versions used by several individual references below -->