[clangd] Add parantheses while auto-completing functions.

[clangd] Add parantheses while auto-completing functions.

Summary:
Currently we only add parantheses to the functions if snippets are
enabled, which also inserts snippets for parameters into parantheses. Adding a
new option to put only parantheses. Also it moves the cursor within parantheses
or at the end of them by looking at whether completion item has any parameters
or not. Still requires snippets support on the client side.

Reviewers: ioeric, ilya-biryukov, hokein

Reviewed By: ioeric

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50835

llvm-svn: 340040