[clangd] Sema ranking tweaks: downrank keywords and injected names.

Summary:
Injected names being ranked too high was just a bug.
The high boost for keywords was intended, but was too much given how useless
keywords are. We should probably boost them on a case-by-case basis eventually.

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48368

llvm-svn: 335723