Reduce enumerator boxing in ProjectElementContainer (#11808)

Fixes #

### Context


### Changes Made


### Testing


### Notes

Author: SimaTian

src/Build/Construction/ProjectElementContainer.cs

@@ -858,7 +858,9 @@ public bool Remove(T item)
                 return false;
             }
 
-            public IEnumerator<T> GetEnumerator() => new Enumerator(_initial, _forwards);
+            public Enumerator GetEnumerator() => new Enumerator(_initial, _forwards);
+
+            IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
 
             IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
@@ -948,13 +950,18 @@ internal ProjectElementSiblingEnumerable(ProjectElement initial, bool forwards =
             /// <summary>
             /// Get enumerator
             /// </summary>
-            public readonly IEnumerator<ProjectElement> GetEnumerator() => _enumerator;
+            public readonly Enumerator GetEnumerator() => _enumerator;
 
             /// <summary>
             /// Get non generic enumerator
             /// </summary>
             IEnumerator IEnumerable.GetEnumerator() => _enumerator;
 
+            /// <summary>
+            /// Get enumerator
+            /// </summary>
+            IEnumerator<ProjectElement> IEnumerable<ProjectElement>.GetEnumerator() => _enumerator;
+
             /// <summary>
             /// Enumerator over a series of sibling ProjectElement objects
             /// </summary>