Debug bootstrap build fails when annotations are added to LocalDataFlowPass

[RikkiGibson]

Version Used: f98dfe5

Steps to Reproduce:

1. Open src\Compilers\CSharp\Portable\FlowAnalysis\LocalDataFlowPass.cs
2. Add #nullable enable annotations to the top of the file
3. Run .\Build.cmd -bootstrap -bootstrapConfiguration:Debug

Expected Behavior:
Bootstrap build completes successfully.

Actual Behavior:
csc crashes with the following stack trace when building Microsoft.CodeAnalysis.CSharp.dll in both netcoreapp3.1 and netstandard2.0 flavors.

Debug.Assert failed with message: Fail: 
Stack Trace
   at Microsoft.CodeAnalysis.CommandLine.ExitingTraceListener.Exit(String originalMessage)
   at Microsoft.CodeAnalysis.CommandLine.ExitingTraceListener.WriteLine(String message)
   at System.Diagnostics.TraceInternal.Fail(String message)
   at System.Diagnostics.Debug.Assert(Boolean condition)
   at Microsoft.Cci.MetadataWriter.HeapOrReferenceIndexBase`1.Add(T item)
   at Microsoft.Cci.MetadataWriter.HeapOrReferenceIndexBase`1.GetOrAdd(T item)
   at Microsoft.Cci.FullMetadataWriter.GetOrAddTypeSpecificationHandle(ITypeReference reference)
   at Microsoft.Cci.MetadataWriter.GetTypeSpecificationHandle(ITypeReference typeReference)
   at Microsoft.Cci.MetadataWriter.GetTypeHandle(ITypeReference typeReference, Boolean treatRefAsPotentialTypeSpec)
   at Microsoft.Cci.MetadataWriter.PopulateGenericParameters(ImmutableArray`1 sortedGenericParameters)
   at Microsoft.Cci.MetadataWriter.PopulateTypeSystemTables(Int32[] methodBodyOffsets, BlobBuilder mappedFieldDataWriter, BlobBuilder resourceWriter, BlobBuilder dynamicAnalysisDataOpt, Blob& mvidFixup)
   at Microsoft.Cci.MetadataWriter.BuildMetadataAndIL(PdbWriter nativePdbWriterOpt, BlobBuilder ilBuilder, BlobBuilder mappedFieldDataBuilder, BlobBuilder managedResourceDataBuilder, Blob& mvidFixup, Blob& mvidStringFixup)
   at Microsoft.Cci.PeWriter.WritePeToStream(EmitContext context, CommonMessageProvider messageProvider, Func`1 getPeStream, Func`1 getPortablePdbStreamOpt, PdbWriter nativePdbWriterOpt, String pdbPathOpt, Boolean metadataOnly, Boolean isDeterministic, Boolean emitTestCoverageData, Nullable`1 privateKeyOpt, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.Compilation.SerializePeToStream(CommonPEModuleBuilder moduleBeingBuilt, DiagnosticBag metadataDiagnostics, CommonMessageProvider messageProvider, Func`1 getPeStream, Func`1 getMetadataPeStreamOpt, Func`1 getPortablePdbStreamOpt, PdbWriter nativePdbWriterOpt, String pdbPathOpt, Boolean metadataOnly, Boolean includePrivateMembers, Boolean isDeterministic, Boolean emitTestCoverageData, Nullable`1 privateKeyOpt, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.Compilation.SerializeToPeStream(CommonPEModuleBuilder moduleBeingBuilt, EmitStreamProvider peStreamProvider, EmitStreamProvider metadataPEStreamProvider, EmitStreamProvider pdbStreamProvider, Func`2 testSymWriterFactory, DiagnosticBag diagnostics, Boolean metadataOnly, Boolean includePrivateMembers, Boolean emitTestCoverageData, String pePdbFilePath, Nullable`1 privateKeyOpt, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CommonCompiler.CompileAndEmit(TouchedFileLogger touchedFilesLogger, Compilation& compilation, ImmutableArray`1 analyzers, ImmutableArray`1 additionalTextFiles, AnalyzerConfigSet analyzerConfigSet, ImmutableArray`1 sourceFileAnalyzerConfigOptions, ImmutableArray`1 embeddedTexts, DiagnosticBag diagnostics, CancellationToken cancellationToken, CancellationTokenSource& analyzerCts, Boolean& reportAnalyzer, AnalyzerDriver& analyzerDriver)
   at Microsoft.CodeAnalysis.CommonCompiler.RunCore(TextWriter consoleOutput, ErrorLogger errorLogger, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CommonCompiler.Run(TextWriter consoleOutput, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CompilerServer.CompilerServerHost.RunCompilation(RunRequest request, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CompilerServer.NamedPipeClientConnection.<>c__DisplayClass17_0.<ServeBuildRequestAsync>b__0()
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean bPreventDoubleExecution)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart(Object obj)