[clangd] Fix crash hovering on non-decltype trailing return

Summary:
More specifically, hovering on "auto" in
auto main() -> int {
    return 0;
}

Signed-off-by: Marc-Andre Laperle <malaperle@gmail.com>

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D54553

llvm-svn: 347119