[clangd] Allow renaming class templates in cross-file rename.

Summary:
It was disabled because we don't handle explicit template
specialization well (due to the index limitation).

renaming templates is normal in practic, rather than disabling it, this patch
allows to rename them though it is not perfect (just a known limitation).

Context: https://github.com/clangd/clangd/issues/280

Reviewers: kbobyrev

Reviewed By: kbobyrev

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74709