C# code generation is not thread-safe

[anpaz]

Describe the bug

Recently, we tried to make a performance optimization on iqsharp to call the codegen asynchronously as we might have multiple block of files (one per snippet) that needs to be compiled. The code can be seen here. In essence:

                foreach (var file in allSources)
                {
                    trees.Add(Task.Run(() =>
                    {
                        var codegenContext = string.IsNullOrEmpty(executionTarget)
                            ? CodegenContext.Create(qsCompilation.Namespaces)
                            : CodegenContext.Create(qsCompilation.Namespaces, new Dictionary<string, string>() { { AssemblyConstants.ExecutionTarget, executionTarget } });
                        var code = SimulationCode.generate(file, codegenContext);
                        return CSharpSyntaxTree.ParseText(code, encoding: UTF8Encoding.UTF8);
                    }));
                }

Unfortunately, we started getting random compilation errors as internal variable names like __arg1__ got reused. The problem is that the code in SimulationCode.fs is not thread-safe. In particular there is a count static variable (line 247 that gets reset when a new Specialization is started (line 1015)[https://github.com/microsoft/qsharp-compiler/blob/main/src/QsCompiler/CSharpGeneration/SimulationCode.fs#L1015]

Expected behavior

That calling SimulationCode.generate is thread-safe.

I believe count is actually the only static variable so we should put that into the context and make sure everything is thread-safe. Most of the work is probably going to be in writing some unittests to gate this behavior.