Add an option to set --limit-results parameter for clangd via the UI

https://bugreports.qt.io/browse/QTCREATORBUG-27152 suggests to use QTC_CLANGD_COMPLETION_RESULTS environment variable but this is not that convenient.

Suggestion is to add an UI option in the clangd property page for this (and plausibly for any other parameters which might be coming). The default behaviour of overriding the clangd command line with a 0 if you don't set this variable even though clangd's default is actually 100 is really not transparent or easy to navigate by the average user. It would be better to have possibility to change this through options in the UI.