Call ImmutableList<T>.AddRange from ConvertAll and FindAll

There are 2 more places where we can benefit from the performance optimization in ImmutableList<T>.AddRange

My benchmarking shows the following results:

    Benchmarking FindAll:
    Using random seed -1979800607
    Optimized; iterations: 20; Time: 00:00:00.3338650
    Original; iterations: 20; Time: 00:00:00.7364930

    Benchmarking ConvertAll:
    Using random seed -1967300607
    Optimized; iterations: 20; Time: 00:00:00.3541609
    Original; iterations: 20; Time: 00:00:01.2586320

(Benchmark code is here: https://github.com/PatrickMcDonald/corefx/tree/immutablelist-addrange-refactor-perf)