[clangd] Index-based code completion.

Summary: Use symbol index to populate completion results for qualfified IDs e.g. "nx::A^".

Reviewers: ilya-biryukov, sammccall

Reviewed By: ilya-biryukov, sammccall

Subscribers: rwols, klimek, mgorny, cfe-commits, sammccall

Differential Revision: https://reviews.llvm.org/D41281

llvm-svn: 321083