Fix typing crash during open file analysis.

My previous PR #37303 to improve IDE open file analysis performance led to a functional regression where attempting to compute open file diagnostics for a partial type leads to recursive attempt to force complete all partial trees, which in turn attempt force completion of original tree. In presence of cancellation request, this leads to a stack overflow and VS crash. The fix here ensures that we don't attempt partial tree analysis when invoked in context of partial analysis itself.
Fixes VSO [#957250](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/957250) and [#957243](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/957243)
Fixes #37567

I have manually verified the crash before the fix and verified it does not repro after the fix. I am working on adding an integration test for the scenario with a follow-up commit/PR.

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

@@ -566,7 +566,7 @@ public async Task<ImmutableArray<Diagnostic>> GetAnalyzerSemanticDiagnosticsAsyn
             return await GetAnalyzerSemanticDiagnosticsCoreAsync(model, filterSpan, analyzers, cancellationToken).ConfigureAwait(false);
         }
 
-        private async Task<ImmutableArray<Diagnostic>> GetAnalyzerSemanticDiagnosticsCoreAsync(SemanticModel model, TextSpan? filterSpan, ImmutableArray<DiagnosticAnalyzer> analyzers, CancellationToken cancellationToken)
+        private async Task<ImmutableArray<Diagnostic>> GetAnalyzerSemanticDiagnosticsCoreAsync(SemanticModel model, TextSpan? filterSpan, ImmutableArray<DiagnosticAnalyzer> analyzers, CancellationToken cancellationToken, bool forceCompletePartialTrees = true)
         {
             try
             {
@@ -588,7 +588,7 @@ private async Task<ImmutableArray<Diagnostic>> GetAnalyzerSemanticDiagnosticsCor
                         cancellationToken.ThrowIfCancellationRequested();
 
                         (ImmutableArray<CompilationEvent> compilationEvents, bool hasSymbolStartActions) = await ComputeAnalyzerDiagnosticsAsync(pendingAnalysisScope, getPendingEvents, taskToken, cancellationToken).ConfigureAwait(false);
-                        if (hasSymbolStartActions)
+                        if (hasSymbolStartActions && forceCompletePartialTrees)
                         {
                             await processPartialSymbolLocationsAsync(compilationEvents, analysisScope).ConfigureAwait(false);
                         }
@@ -649,7 +649,7 @@ async Task processPartialSymbolLocationsAsync(ImmutableArray<CompilationEvent> c
                             Task.Run(() =>
                             {
                                 var treeModel = _compilationData.GetOrCreateCachedSemanticModel(tree, _compilation, cancellationToken);
-                                return GetAnalyzerSemanticDiagnosticsCoreAsync(treeModel, filterSpan: null, analysisScope.Analyzers, cancellationToken);
+                                return GetAnalyzerSemanticDiagnosticsCoreAsync(treeModel, filterSpan: null, analysisScope.Analyzers, cancellationToken, forceCompletePartialTrees: false);
                             }, cancellationToken))).ConfigureAwait(false);
                     }
                     else
@@ -658,7 +658,7 @@ async Task processPartialSymbolLocationsAsync(ImmutableArray<CompilationEvent> c
                         {
                             cancellationToken.ThrowIfCancellationRequested();
                             var treeModel = _compilationData.GetOrCreateCachedSemanticModel(tree, _compilation, cancellationToken);
-                            await GetAnalyzerSemanticDiagnosticsCoreAsync(treeModel, filterSpan: null, analysisScope.Analyzers, cancellationToken).ConfigureAwait(false);
+                            await GetAnalyzerSemanticDiagnosticsCoreAsync(treeModel, filterSpan: null, analysisScope.Analyzers, cancellationToken, forceCompletePartialTrees: false).ConfigureAwait(false);
                         }
                     }
                 }