[clangd] Fix a crash in Selection

Summary:
The assertion checking that a range of a node is a token range does
not hold in case of "split" tokens, e.g. between two closing template
argument lists (`vector<vector<int>>`).

Reviewers: kadircet, sammccall

Reviewed By: kadircet

Subscribers: ioeric, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58447

llvm-svn: 354507