Add LangVersion to _GenerateCompileDependencyCache (#9656)

Include LangVersion as `CoreCompileCache` to invalidate incremental build in the case when
building with `dotnet build -p:langversion=X` then with `dotnet build -p:langversion=Y`.

Fixes dotnet/roslyn#71678

src/Tasks/Microsoft.Common.CurrentVersion.targets

@@ -3799,6 +3799,7 @@ Copyright (C) Microsoft Corporation. All rights reserved.
       <CoreCompileCache Include="@(Compile)" />
       <CoreCompileCache Include="@(ReferencePath)" />
       <CoreCompileCache Include="$(DefineConstants)" />
+      <CoreCompileCache Include="$(LangVersion)" />
     </ItemGroup>
 
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