[clangd] Handle the missing call expr in targetDecl.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: merge_guards_bot, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70773