[clangd] Support offsets for parameters in signatureHelp

Summary: Added to LSP in version 3.14

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62476

llvm-svn: 362481