Add a setting to stop automatically tag parsing all headers under the current workspace folder

[sean-mcmanus]

Currently, headers under the workspace folder are always tag parsed, even if they're not #included. This decision was made before multi-root workspaces were possible in which users could add large external libraries with lots of headers that aren't used, but can increase the database size and decrease performance (see #6801).

Most of the work involved is just adding the setting and plumbing it through, similar to what is already done for the limitSymbolsToIncludedHeaders setting.

[bobbrow]

I don't think we want to do this. At some point we'd like to have a feature that will allow us to suggest types that have not yet been #included. Auto-completing those types would also add the #include statement.

[sean-mcmanus]

Sure, I'll close this.