[clangd] Find reference to template parameter in 'sizeof...' expression

Fixes https://github.com/clangd/clangd/issues/213.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70740