[clangd] Extract per-dir CDB cache to its own threadsafe class. NFC

This is a step towards making compile_commands.json reloadable.

The idea is:
 - in addition to rare CDB loads we're soon going to have somewhat-rare CDB
   reloads and fairly-common stat() of files to validate the CDB
 - so stop doing all our work under a big global lock, instead using it to
   acquire per-directory structures with their own locks
 - each directory can be refreshed from disk every N seconds, like filecache
 - avoid locking these at all in the most common case: directory has no CDB

Differential Revision: https://reviews.llvm.org/D92381

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