[source maps] Fix remove, insert-after and replace

Summary:
In this diff of mine D77186 I introduce a bug in the replace operation, where I was failing fast by mistake.
Besides, a similar problem existed in the insert-after operation, where it was failing fast.

Finally, the remove operation was wrong, as it was not using the indices provided by the users.

I fixed those issues and added some tests account for cases with multiple elements in these requests.

Reviewers: labath, clayborg

Reviewed By: labath

Subscribers: mgrang, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D77324