[clangd] Store explicit template specializations in index for code navigation purposes

Summary:
This introduces ~4k new symbols, and ~10k refs for LLVM. We need that
information for providing better code navigation support:
- When references for a class template is requested, we should return these specializations as well.
- When children of a specialization is requested, we should be able to query for those symbols(instead of just class template)

Number of symbols: 378574 -> 382784
Number of refs: 5098857 -> 5110689

Reviewers: hokein, gribozavr

Reviewed By: gribozavr

Subscribers: nridge, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59083

llvm-svn: 356125