[clangd] Code completion: drop explicit injected names/operators, ignore Sema priority

Summary:
Now we have most of Sema's code completion signals incorporated in Quality,
which will allow us to give consistent ranking to sema/index results.

Therefore we can/should stop using Sema priority as an explicit signal.
This fixes some issues like namespaces always having a terrible score.

The most important missing signals are:
 - Really dumb/rarely useful completions like:
    SomeStruct().^SomeStruct
    SomeStruct().^operator=
    SomeStruct().~SomeStruct()
   We already filter out destructors, this patch adds injected names and
   operators to that list.
 - type matching the expression context.
   Ilya has a plan to add this in a way that's compatible with indexes
   (design doc should be shared real soon now!)

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D47871

llvm-svn: 334192