[clangd] Support multiple cursors in selectionRange.

Summary:
One change: because there's no way to signal failure individually for
each cursor, we now "succeed" with an empty range with no parent if a
cursor doesn't point at anything.

Reviewers: usaxena95

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76741