C++: handle case-insensitive file names in the CPlusPlus::Snapshot

... by keying on Utils::FileName

Task-number: QTCREATORBUG-12390
Change-Id: Ia98afb5a9160a7fd9225a2f9e02539ff3c35ae86
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>