[clangd] Add activate command to the vscode extension.

Summary:
This would help minizime the annoying part of not activating the extension
for .cu file.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59817

llvm-svn: 357075