Separate variance information in mangled names (dotnet#63866)

Fixes dotnet/runtimelab#1821.

Eventually we'll need to have a holistic look at how we mangle things.

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/DependencyAnalysis/GenericCompositionNode.cs

@@ -36,6 +36,7 @@ public void AppendMangledName(NameMangler nameMangler, Utf8StringBuilder sb)
 
             if (_details.Variance != null)
             {
+                sb.Append("__Variance__");
                 for (int i = 0; i < _details.Variance.Length; i++)
                 {
                     sb.Append('_');