[clangd] Don't rebuild background index until we indexed one TU per thread.

Summary:
This increases the odds that the boosted file (cpp file matching header)
will be ready. (It always enqueues first, so it'll be present unless
another thread indexes *two* files before the first thread indexes one.)

Reviewers: kadircet

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64682

llvm-svn: 366199