[clangd] NFC: Prefer isa<> to dyn_cast<> to do the checking.

Summary: Prefer isa<> to dyn_cast<> when there only need a checking.

Reviewers: ilya-biryukov, MaskRay

Reviewed By: ilya-biryukov, MaskRay

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

Differential Revision: https://reviews.llvm.org/D54878