[clangd] Minimal implementation of automatic static index (not enabled).

Summary:
See tinyurl.com/clangd-automatic-index for design and goals.

Lots of limitations to keep this patch smallish, TODOs everywhere:
 - no serialization to disk
 - no changes to dynamic index, which now has a much simpler job
 - no partitioning of symbols by file to avoid duplication of header symbols
 - no reindexing of edited files
 - only a single worker thread
 - compilation database is slurped synchronously (doesn't scale)
 - uses memindex, rebuilds after every file (should be dex, periodically)

It's not hooked up to ClangdServer/ClangdLSPServer yet: the layering
isn't clear (it should really be in ClangdServer, but ClangdLSPServer
has all the CDB interactions).

Reviewers: ioeric

Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D53032

llvm-svn: 344513