[clangd] Auto-index watches global CDB for changes.

Summary:
Instead of receiving compilation commands, auto-index is triggered by just
filenames to reindex, and gets commands from the global comp DB internally.
This has advantages:
 - more of the work can be done asynchronously (fetching compilation commands
   upfront can be slow for large CDBs)
 - we get access to the CDB which can be used to retrieve interpolated commands
   for headers (useful in some cases where the original TU goes away)
 - fits nicely with the filename-only change observation from r347297

The interface to GlobalCompilationDatabase gets extended: when retrieving a
compile command, the GCDB can optionally report the project the file belongs to.
This naturally fits together with getCompileCommand: it's hard to implement one
without the other. But because most callers don't care, I've ended up with an
awkward optional-out-param-in-virtual method pattern - maybe there's a better
one.

This is the main missing integration point between ClangdServer and
BackgroundIndex, after this we should be able to add an auto-index flag.

Reviewers: ioeric, kadircet

Subscribers: MaskRay, jkorous, arphaman, cfe-commits, ilya-biryukov

Differential Revision: https://reviews.llvm.org/D54865

llvm-svn: 347538