[clangd] Fix a crash bug in AddUsing tweak around template handling.

Summary:
The crash happened on cases like:
template<typename TT> using one = two::three<T^T>;
because we tried to call getName() on getBaseTypeIdentifier(), which can
be nullptr.

Ideally we would support this use case as well, but for now not crashing
will do.

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77656