[clangd] Add a test for SignatureHelp on dynamic index.

Summary: This would catch regressions caused by future changes of the index.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D56483

llvm-svn: 350720