[clangd] Only reduce priority of a thread for indexing.

Summary:
We'll soon have tasks pending for reading shards from disk, we want
them to have normal priority. Because:
- They are not CPU intensive, mostly IO bound.
- Give a good coverage for the project at startup, therefore it is worth
  spending some cycles.
- We have only one task per whole CDB rather than one task per file.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D55315

llvm-svn: 349345