[clangd] Use the clang-tools-extra as the official repo for `vscode-clangd` extension.

Summary:
Previously, we use a separate GitHub repository (https://github.com/llvm-vs-code-extensions/vscode-clangd)
for publishing `vscode-clangd` extension to marketplace.

To reduce the maintain burden, we will use the vscode extension in the
clang-tools-extra, and deprecate the one on GitHub.

Test in https://marketplace.visualstudio.com/items?itemName=llvm-vs-code-extensions.clangd-vscode-test

Reviewers: sammccall, krasimir

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41391

llvm-svn: 321252