Pre-size intermediate Dictionary in ToFrozenDictionary

When the source is not already a Dictionary, ToFrozenDictionary builds
an intermediate Dictionary to deduplicate keys with last-wins semantics.
The previous code used the default-capacity constructor, which paid
log2(N) resizes and ~2N redundant rehashes when populating from a known
size source such as an array or List.

Use ICollection<KeyValuePair>.Count as a capacity hint when available,
mirroring the HashSet(IEnumerable, IEqualityComparer) constructor.
The ReadOnlySpan overload of FrozenDictionary.Create already pre-sizes,
so this brings the IEnumerable path in line with it.

Author: AndrewP-GH

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/FrozenDictionary.cs

@@ -131,7 +131,8 @@ public static FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey,
             newDictionary = source as Dictionary<TKey, TValue>;
             if (newDictionary is null || (newDictionary.Count != 0 && !newDictionary.Comparer.Equals(comparer)))
             {
-                newDictionary = new Dictionary<TKey, TValue>(comparer);
+                int capacity = source is ICollection<KeyValuePair<TKey, TValue>> collection ? collection.Count : 0;
+                newDictionary = new Dictionary<TKey, TValue>(capacity, comparer);
                 foreach (KeyValuePair<TKey, TValue> pair in source)
                 {
                     // Dictionary's constructor uses Add, which will throw on duplicates.