[clangd] Query index in code completion no-compile mode.

Summary: We scrape the enclosing scopes from the source file, and use them in the query.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61077

llvm-svn: 359284