ToImmutable cache roots objects between calls

[sharwell]

Steps to reproduce:

1. Create an ImmutableList<string> of strings
2. Get a builder for the immutable list
3. Call ImmutableList<string>.Builder.Clear()

Expected:

The strings are eligible for GC.

Actual:

The ImmutableList<T>.Builder._immutableList field keeps the strings rooted until it is reset the next time ToImmutable() is called.

Possible solutions:

1. Clear the _immutableList field each time _version is incremented
2. Use a WeakReference<T> instead of storing _immutableList directly