[clangd] Fix no results returned for global symbols in dexp

Summary:
For symbols in global namespace (without any scope), we need to
add global scope "" to the fuzzy request.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D54519

llvm-svn: 346947