[Index] Compute correct symbol kind for variable templates

Summary:
The index library itself seems to never pass variable templates as
input, however clangd does.

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62579

llvm-svn: 361996