Bump Microsoft.CodeAnalysis.CSharp from 4.0.1 to 4.3.1 (#622)

Bumps [Microsoft.CodeAnalysis.CSharp](https://github.com/dotnet/roslyn) from 4.0.1 to 4.3.1.
- [Release notes](https://github.com/dotnet/roslyn/releases)
- [Changelog](https://github.com/dotnet/roslyn/blob/main/docs/Breaking%20API%20Changes.md)
- [Commits](https://github.com/dotnet/roslyn/commits)

---
updated-dependencies:
- dependency-name: Microsoft.CodeAnalysis.CSharp
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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src/CodeGeneration/CodeGeneration.csproj

@@ -5,7 +5,7 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.0.1" />
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.3.1" />
     <PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.3.3" PrivateAssets="all" />
     <PackageReference Include="Nullable" Version="1.3.0">
       <PrivateAssets>all</PrivateAssets>