Fix intermittent crash in CompilationWithAnalyzers due to an OjbectDisposedException.

Fixes VSO Bug [199252](https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?_a=edit&id=199252)

Root cause: We were attempting to access the cancellation token for a cancellation token source within a lambda within the using statement for the token source. If client fires a cancellation before the task within the lambda executes, the token source may have been disposed when the task is eventually activated.

Fix: This change saves the cancellation token for the token source upfront for use within the lambda and also checks for cancellation before computing diagnostics.

Justification for ask mode: We are hitting this intermittent crash in perf runs in an internal VSO branch and fix doesn't have any risks.

src/Compilers/Core/Portable/DiagnosticAnalyzer/CompilationWithAnalyzers.cs

@@ -559,6 +559,9 @@ private async Task ComputeAnalyzerDiagnosticsAsync(AnalysisScope analysisScope,
                             {
                                 try
                                 {
+                                    // Fetch the cancellation token here to avoid capturing linkedCts in the getComputeTask lambda as the task may run after linkedCts has been disposed due to cancellation.
+                                    var linkedCancellationToken = linkedCts.Token;
+
                                     // Core task to compute analyzer diagnostics.
                                     Func<Tuple<Task, CancellationTokenSource>> getComputeTask = () => Tuple.Create(
                                         Task.Run(async () =>
@@ -571,8 +574,10 @@ private async Task ComputeAnalyzerDiagnosticsAsync(AnalysisScope analysisScope,
                                                     // Get event queue with pending events to analyze.
                                                     eventQueue = getEventQueue();
 
+                                                    linkedCancellationToken.ThrowIfCancellationRequested();
+
                                                     // Execute analyzer driver on the given analysis scope with the given event queue.
-                                                    await ComputeAnalyzerDiagnosticsCoreAsync(driver, eventQueue, analysisScope, cancellationToken: linkedCts.Token).ConfigureAwait(false);
+                                                    await ComputeAnalyzerDiagnosticsCoreAsync(driver, eventQueue, analysisScope, cancellationToken: linkedCancellationToken).ConfigureAwait(false);
                                                 }
                                                 finally
                                                 {
@@ -584,7 +589,7 @@ private async Task ComputeAnalyzerDiagnosticsAsync(AnalysisScope analysisScope,
                                                 throw ExceptionUtilities.Unreachable;
                                             }
                                         },
-                                            linkedCts.Token),
+                                            linkedCancellationToken),
                                         cts);
 
                                     // Wait for higher priority tree document tasks to complete.