ProjectRootElementCache.Clear do not clear immutable files (#8540)

For `msbuild -restore` scenarios `ProjectRootElementCache` cache used to be cleared for restore could have modified or added some project files (like *.directory.props etc...). 

We can optimize it by keeping ProjectRootElements from immutable locations (like SDK) in that cache as those will be used by build following that restore.

src/Build/Evaluation/ProjectRootElementCache.cs

@@ -7,6 +7,7 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
 using System.IO;
+using System.Linq;
 using System.Xml;
 using Microsoft.Build.Collections;
 using Microsoft.Build.Construction;
@@ -458,13 +459,44 @@ namespace Microsoft.Build.Evaluation
         {
             lock (_locker)
             {
-                LinkedList<ProjectRootElement> oldStrongCache = _strongCache;
-                _weakCache = new WeakValueDictionary<string, ProjectRootElement>(StringComparer.OrdinalIgnoreCase);
-                _strongCache = new LinkedList<ProjectRootElement>();
-
-                foreach (ProjectRootElement projectRootElement in oldStrongCache)
+                if (Traits.Instance.EscapeHatches.AlwaysDoImmutableFilesUpToDateCheck)
                 {
-                    RaiseProjectRootElementRemovedFromStrongCache(projectRootElement);
+                    LinkedList<ProjectRootElement> oldStrongCache = _strongCache;
+                    _weakCache = new WeakValueDictionary<string, ProjectRootElement>(StringComparer.OrdinalIgnoreCase);
+                    _strongCache = new LinkedList<ProjectRootElement>();
+
+                    foreach (ProjectRootElement projectRootElement in oldStrongCache)
+                    {
+                        RaiseProjectRootElementRemovedFromStrongCache(projectRootElement);
+                    }
+                }
+                else
+                {
+                    // Manually iterate through LinkedList so we can remove items during this iteration
+                    for (var listNode = _strongCache.First; listNode != null;)
+                    {
+                        var nextNode = listNode.Next;
+
+                        ProjectRootElement projectRootElement = listNode.Value;
+                        // Do not remove cache of files from immutable locations.
+                        // Those are mostly SDK project files and will be most probably needed in next builds.
+                        if (!FileClassifier.Shared.IsNonModifiable(projectRootElement.FullPath))
+                        {
+                            _weakCache.Remove(projectRootElement.FullPath);
+                            _strongCache.Remove(listNode);
+                            RaiseProjectRootElementRemovedFromStrongCache(projectRootElement);
+                        }
+
+                        listNode = nextNode;
+                    }
+
+                    // From weak list remove all which is not in strong list anymore
+                    IList<string> toBeRemovedFromWeakRefs = _weakCache.Keys.Except(_strongCache.Select(i => i.FullPath)).ToList();
+                    foreach (string victim in toBeRemovedFromWeakRefs)
+                    {
+                        _weakCache.Remove(victim);
+                    }
+                    _weakCache.Scavenge();
                 }
             }
         }