Add asserts to help diagnose signature help crash #832

Bug #832 is a crash that I cannot reproduce. There are a few plausible causes for this crash, so I'm adding a few asserts to catch these cases in an effort to learn more. These asserts should not be fatal, and they should not degrade the user experience.

The basic problem is that we are looking at the children of a call expression, trying to find a list child that contains a given span. The 2 asserts I have do the following:
1. Assert that as we walk up the tree, every node's span is contained in its parent's span
2. If we did not find list children containing the span of interest, assert that we can at least find list children.