[clangd] Don't index template specializations.

Summary:
These have different USRs than the underlying entity, but are not typically
interesting in their own right and can be numerous (e.g. generated traits).

Reviewers: ioeric

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D44298

llvm-svn: 327127