Use PooledHashSet to avoid extra allocations

Many of these can be called fairly often, so this avoids allocations in
the common case we don't refresh anything.

src/VisualStudio/Core/Def/Implementation/ProjectSystem/VisualStudioWorkspaceImpl.cs

@@ -13,6 +13,7 @@
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
 using Microsoft.CodeAnalysis.Host;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Shared.Utilities;
 using Microsoft.CodeAnalysis.SolutionCrawler;
 using Microsoft.CodeAnalysis.Text;
@@ -1421,7 +1422,7 @@ public void AddProjectOutputPath(ProjectId projectId, string outputPath)
         private void ConvertMetadataReferencesToProjectReferences_NoLock(ProjectId projectId, string outputPath)
         {
             var modifiedSolution = this.CurrentSolution;
-            var projectIdsChanged = new HashSet<ProjectId>();
+            var projectIdsChanged = PooledHashSet<ProjectId>.GetInstance();
 
             foreach (var projectIdToRetarget in this.CurrentSolution.ProjectIds)
             {
@@ -1452,6 +1453,7 @@ private void ConvertMetadataReferencesToProjectReferences_NoLock(ProjectId proje
             }
 
             SetSolutionAndRaiseWorkspaceChanged_NoLock(modifiedSolution, projectIdsChanged);
+            projectIdsChanged.Free();
         }
 
         [PerformanceSensitive("https://github.com/dotnet/roslyn/issues/31306",
@@ -1489,7 +1491,7 @@ private bool CanConvertMetadataReferenceToProjectReference(ProjectId projectIdWi
         private void ConvertProjectReferencesToMetadataReferences_NoLock(ProjectId projectId, string outputPath)
         {
             var modifiedSolution = this.CurrentSolution;
-            var projectIdsChanged = new HashSet<ProjectId>();
+            var projectIdsChanged = PooledHashSet<ProjectId>.GetInstance();
 
             foreach (var projectIdToRetarget in this.CurrentSolution.ProjectIds)
             {
@@ -1521,6 +1523,7 @@ private void ConvertProjectReferencesToMetadataReferences_NoLock(ProjectId proje
             }
 
             SetSolutionAndRaiseWorkspaceChanged_NoLock(modifiedSolution, projectIdsChanged);
+            projectIdsChanged.Free();
         }
 
         public ProjectReference TryCreateConvertedProjectReference(ProjectId referencingProject, string path, MetadataReferenceProperties properties)
@@ -1634,7 +1637,7 @@ private void RefreshMetadataReferencesForFile(object sender, string fullFilePath
             lock (_gate)
             {
                 var newSolution = CurrentSolution;
-                var changedProjectIds = new HashSet<ProjectId>();
+                var changedProjectIds = PooledHashSet<ProjectId>.GetInstance();
 
                 foreach (var project in CurrentSolution.Projects)
                 {
@@ -1662,6 +1665,7 @@ private void RefreshMetadataReferencesForFile(object sender, string fullFilePath
                 }
 
                 SetSolutionAndRaiseWorkspaceChanged_NoLock(newSolution, changedProjectIds);
+                changedProjectIds.Free();
             }
         }
     }