[clangd] Move function argument snippet disable mechanism from LSP rendering...

[clangd] Move function argument snippet disable mechanism from LSP rendering to internal clangd reprensentation.

Summary:
We were handling the EnableFunctionArgSnippets only when we are producing LSP
response. Move that code into CompletionItem generation so that internal clients
can benefit from that as well.

Reviewers: ilya-biryukov, ioeric, hokein

Reviewed By: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D51102

llvm-svn: 340527