[clangd] Fix the incomplete template specialization in findTarget.

Summary:
FindTarget doesn't report the TemplatePattern for incomplete
specialization.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74900