C#: Upgrade to Roslyn 3.1

Author: calumgrant

csharp/extractor/Semmle.Extraction.CSharp/Semmle.Extraction.CSharp.csproj

@@ -19,7 +19,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.0.0" />
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.1.0" />
   </ItemGroup>
 
 </Project>

csharp/extractor/Semmle.Extraction/Semmle.Extraction.csproj

@@ -12,7 +12,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis" Version="3.0.0" />
+    <PackageReference Include="Microsoft.CodeAnalysis" Version="3.1.0" />
     <PackageReference Include="GitInfo" Version="2.0.18" />
   </ItemGroup>