[clangd] Suppress unwritten scopes when expanding auto.

Summary: otherwise the replacement will break the code.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64627

llvm-svn: 366446