[clangd] Move DirBasedCDB broadcasting onto its own thread.

This is on the critical path (it blocks getting the compile command for
the first file).

It's not trivially fast: it involves processing all filenames in the CDB
and doing some IO to look for shadowing CDBs.

And we may make this slower soon - making CDB configurable implies evaluating
the config for each listed to see which ones really are owned by the
broadcasted CDB.

Differential Revision: https://reviews.llvm.org/D94606