ChangedText.WithChanges crashes when new deletion larger than old

[RikkiGibson]

Do we also need to handle the case where the new deletion is larger than the old? It looks like the following fails with or without this PR:

var original = SourceText.From("01234");
var change1 = original.WithChanges(new TextChange(new TextSpan(1, 3), "aa"));
var change2 = change1.WithChanges(new TextChange(new TextSpan(1, 3), "bb"));

var changes = change2.GetTextChanges(original);
Assert.Equal("0aa4", change1.ToString());
Assert.Equal("0bb", change2.ToString());

We can log a bug for this case and handle it separately.

Originally posted by @cston in #39258

/cc @ashmind