[clangd] Add option to enable/disable function argument snippets.

Summary:
Currently LSP clients cannot directly change EnableFunctionArgSnippets parameter.
This patch is to provide them with a way to enable/disable that functionality.

Reviewers: hokein, ioeric, ilya-biryukov

Reviewed By: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D51214

llvm-svn: 342533