Fix cancellation handling in Parallel.For

This change ensures that cancellation exceptions that get wrapped in an
AggregateException are treated as cancellation instead of failure.

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -2489,6 +2489,13 @@ internal override void GetDiagnostics(CompilationStage stage, bool includeEarlie
                             {
                                 throw ExceptionUtilities.Unreachable;
                             }
+                            catch (OperationCanceledException e) when (cancellationToken.IsCancellationRequested && e.CancellationToken != cancellationToken)
+                            {
+                                // Parallel.For checks for a specific cancellation token, so make sure we throw with the
+                                // correct one.
+                                cancellationToken.ThrowIfCancellationRequested();
+                                throw ExceptionUtilities.Unreachable;
+                            }
                         }));
                 }
                 else

src/Compilers/CSharp/Portable/Symbols/Source/SourceNamespaceSymbol_Completion.cs

@@ -64,6 +64,13 @@ internal override void ForceComplete(SourceLocation locationOpt, CancellationTok
                                     {
                                         throw ExceptionUtilities.Unreachable;
                                     }
+                                    catch (OperationCanceledException e) when (cancellationToken.IsCancellationRequested && e.CancellationToken != cancellationToken)
+                                    {
+                                        // Parallel.For checks for a specific cancellation token, so make sure we throw with the
+                                        // correct one.
+                                        cancellationToken.ThrowIfCancellationRequested();
+                                        throw ExceptionUtilities.Unreachable;
+                                    }
                                 }));
 
                                 foreach (var member in members)

src/Compilers/VisualBasic/Portable/Compilation/VisualBasicCompilation.vb

@@ -2094,6 +2094,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                                 builder.AddRange(SyntaxTrees(i).GetDiagnostics(cancellationToken))
                             Catch e As Exception When FatalError.ReportUnlessCanceled(e)
                                 Throw ExceptionUtilities.Unreachable
+                            Catch e As OperationCanceledException When cancellationToken.IsCancellationRequested AndAlso e.CancellationToken <> cancellationToken
+                                ' Parallel.For checks for a specific cancellation token, so make sure we throw with the
+                                ' correct one.
+                                cancellationToken.ThrowIfCancellationRequested()
+                                Throw ExceptionUtilities.Unreachable
                             End Try
                         End Sub))
                 Else

src/Compilers/VisualBasic/Portable/Symbols/Source/SourceModuleSymbol.vb

@@ -625,6 +625,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
                                 TryGetSourceFile(trees(i)).GenerateAllDeclarationErrors()
                             Catch e As Exception When FatalError.ReportUnlessCanceled(e)
                                 Throw ExceptionUtilities.Unreachable
+                            Catch e As OperationCanceledException When cancellationToken.IsCancellationRequested AndAlso e.CancellationToken <> cancellationToken
+                                ' Parallel.For checks for a specific cancellation token, so make sure we throw with the
+                                ' correct one.
+                                cancellationToken.ThrowIfCancellationRequested()
+                                Throw ExceptionUtilities.Unreachable
                             End Try
                         End Sub))
                 trees.Free()