#1592 clangd_client: add option to parse only opened files

[Christo]

Patch adds an option in clangd_client parser preferences to add an option to parse only open files.

With old laptops, if we allow all files of a large project to be parsed, laptop becomes unusable because of high load for a long time. This patch gives an option to avoid this, but if selected, it will affect "Find defintion" and "Find references" features as it would be done only on a subset of files.

I've been using this in an old laptop for a while and this is quite helpful.

[pecan]

Great addition, thanks ! I'll be using this even on a new powerful laptop.
Applied SVN rev 13829