[clangd] Add support for snippet completions

Enhances CompletionItemsCollector in such a way that snippet
completions can be presented to the client. Enable snippet completion
items by specifying -enable-snippets while invoking the clangd
executable.

See: https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md#completion-request
See: https://github.com/Microsoft/vscode/blob/master/src/vs/editor/contrib/snippet/browser/snippet.md

Patch by Raoul Wols.

llvm-svn: 313029