Stop boxing WeakReferenceListEnumerator in PresentationSource use (#6502)

PresentationSource.CriticalCurrentSources is internal and is only ever foreach'd by consumers.  It's strongly-typed to return IEnumerable, but the actual WeakReferenceList it returns has a struct-based enumerator, which means each foreach boxes that enumerator.  By changing CriticalCurrentSources to return `WeakReferenceList` rather than `IEnumerable`, those foreach's can bind directly to the struct, with all the benefits that brings.

Author: stephentoub

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/PresentationSource.cs

@@ -661,11 +661,11 @@ internal static bool UnderSamePresentationSource(params DependencyObject[] visua
         ///   over a ReadOnly SnapShot of the List of sources.  The Enumerator
         ///   skips over the any dead weak references in the list.
         /// </summary>
-        internal static IEnumerable CriticalCurrentSources
+        internal static WeakReferenceList CriticalCurrentSources
         {
             get
             {
-                return (IEnumerable)_sources;
+                return _sources;
             }
         }