[clang][Index] Fix the incomplete instantiations in libindex.

Summary:
libindex will canonicalize references to template instantiations:
- 1) reference to an explicit template specialization, report the specializatiion
- 2) otherwise, report the primary template

but 2) is not true for incomplete instantiations, this patch fixes this.

Fixes https://github.com/clangd/clangd/issues/287

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74830