Try to keep track of the correct locations whilst editing files

[mpickering]

The language server is a bit useless as soon as you start editing files as all the locations are immediately incorrect. This means all the hovers are in the wrong place and so on.

This patch tries to work out the correct mapping from the old to new locations so that you can edit a file and the ranges are resolved correctly.

The logic is based on a Haskell implementation of the same idea by Luke Lau (@bubba) and Zubin Duggal (@wz1000)

[msftclas]

All CLA requirements met.

[dbaeumer]

@mpickering this is actually an interesting idea. The only problem I see with it is that it might be totally unclear when presented information is actually out of date. Can you elabortate a little more in which use cases something like this is useful? Our current use case for LSIF is more browsing of read only code.

[mpickering]

I was originally using LSIF for a project which couldn't be used with a language server but I could generate LSIF files for. It is very big so most of the files are never modified so LSIF files generally worked fine for browsing those files and finding references. When editing a file I realised that it made everything in the wrong place even though 90% of the file was not modified so this change makes the LSIF files work still for the rest of the file which hasn't been modified.

In recent developments I made said project work with a language server so I have less use for this but I can still imagine using LSIF in a code review situation to make some changes to the patch and suggest them to the author.

[dbaeumer]

@mpickering thanks for the clarification.