[clangd] Do not insert parentheses when completing a using declaration

Summary:
Would be nice to also fix this in clang, but that looks like more work
if we want to preserve signatures in informative chunks.

Fixes https://github.com/clangd/clangd/issues/118

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: merge_guards_bot, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69382