[clangd] Handle the missing injectedClassNameType in targetDecl.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73102

(cherry picked from commit 5d4e8997)