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