This adds a new response element to gotodefinition responses: SourceGeneratedFileInfo. This is similar to MetadataSource, except that unlike MetadataSource it's not tracked on a type/project basis, but rather as a document/project basis. Retrieving info about a source generated file can be done through the SourceGeneratedFileService endpoints:

* SourceGeneratedFileInfo - Gets the file content of a source generated file.
* UpdateSourceGeneratedFileInfo - Gets the updated content of a source generated file, if it has changed since the last time information was returned.
* SourceGeneratedFileClosed - Sent to the server to inform it that the editor has closed the generated file and it can stop tracking Document version info for that file.

Currently, the only way to get the info needed to open a source-generated file is via the gotodefinition endpoint. We'll want to add info to find-usages as well, but that's a job for another day. Contributes to OmniSharp#1934.