[clangd] Initial supoprt for cross-namespace global code completion.

Summary:
When no scope qualifier is specified, allow completing index symbols
from any scope and insert proper automatically. This is still experimental and
hidden behind a flag.

Things missing:
- Scope proximity based scoring.
- FuzzyFind supports weighted scopes.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: kbobyrev, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52364

llvm-svn: 343248