Don't add 'override' specifier for non-modern project settings

Summary:
Override code completion will only add the override specifier if the parser
settings don't have either -std=c++03 or 98.

Supersedes: D4039

BUG: 372280

Test Plan: Run `test_codecompletion testOverrideExecute`

Reviewers: kfunk

Reviewed By: kfunk

Subscribers: kfunk, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D16773