[clangd] Implement "prepareRename"

Summary:
- "prepareRename" request is added in LSP v3.12.0
- also update the vscode-client dependency to pick-up the rename bug fix[1]

[1]: https://github.com/microsoft/vscode-languageserver-node/issues/447

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63126

llvm-svn: 366873