Add lint rule for UnusedTaskIntent

[jabraham17]

Adds a new lint rule UnusedTaskIntent. This is similar to UnusedFormal and UnusedLoopIndex and warns for unused task intents like the following

var a = 1;
begin with (ref a) { }

Since a is not used, it can be safely removed.

Note that the following is not warning for, since task private variables could have side effects

begin with (var x = ...) { }

This PR also fixes an issue where @chplcheck.ignore was being added to AstNode's that did not support attributes for the IncorrectIndentation rule

Future work: Improve the naming of sub-location method names like block_header and header_location

[Reviewed by @DanilaFe]

[DanilaFe]

Looks good but in retrospect I think we need to fix up / make more consistent the names of the various additional location fns ("block header" vs "header location" is confusing to me here.