Support ignore patterns for the cppeditor indexer

I have developed support for ignore patterns for the cppeditor's indexer. Patch is attached.

The reason why I need this is because I have at an organisation that I do work for with qtcreator a large monolithic project that uses cmake. We only need to work on some of the targets. Yet all of the target's files get indexed. This makes qtcreator's C++ indexing very time consuming (and annoying).

Gerrit: https://codereview.qt-project.org/c/qt-creator/qt-creator/+/438366