Figure out how to do background indexing for projects with a compilation database

[ahoppen]

I’m still not sure what we want to do, but we need to figure out if there’s any way in which we can support background indexing for projects with compilation database.

rdar://127474864

[ahoppen]

Compilation databases are missing the information necessary to prepare targets and thus provide background indexing. SourceKit-LSP exposes its build system integration to external build systems via the build server protocol now, as described in Implementing a BSP server.md. If a build system wants to provide background indexing using SourceKit-LSP, it should implement a BSP server.