[find-all-symbols] Index partial template specializations.

Summary:
Fix no std::function index.

Previously, we don't index all the template specialization declarations of functions and classes (Because we assume that template functions/classes are indexed by their template declarations), but this is not always true in some cases like `std::function` which only has a partial template specialization declaration.

Reviewers: ioeric, bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D27920

llvm-svn: 291669