ImmutableArray<T>.Builder.Sort should not create Comparer instance

ImmutableArray<T>.Builder.Sort should not create Comparer instance if count<=1

Tweaking Sort(int index, int count, IComparer<T> comparer) to check for count instead of Count. Checking for count > 1 will ensure both of these
i. The no. of elements intended to be sorted is > 1
ii. The underlying array contains > 1 elements