Teach code-indexer to index calls to functions
It would be really handy if we had an index for all the locations a function is called. We should be able to do this by looking for Call cursors in clang. We'll also need a new type for the IdeCodeIndexEntry to denote it's a call and then put it into a separate index.
It will also change the flags we can use when indexing C files. (We'll have to start indexing function bodies again).
That will allow our code-index symbol resolver to implement FindAllReferences.