clang-format: penalty for breaking double brackets for c++ attributes

[mdavis199]

There should be a penalty setting that prevents breaking the double-brackets around c++ attributes.

Using the attached clang-format, the following code:

int PluginColumnModel::columnCount([[maybe_unused]] const QModelIndex& parent) const {}

is formated as

int PluginColumnModel::columnCount([
    [maybe_unused]] const QModelIndex& parent) const {}

clang-format.txt

[rymiel]

Fixed by 5a4ddbd

There probably doesn't need to be a config option for this because I'd consider the two square brackets ever being broken up as a bug