language: Structure-pack most used types

Make use of C++11 typed-enums and C++11 unions

sizeof-statistics (64 bit system):

• DUChainBaseData: 24B -> 20B
• AbstractTypeData: 32B -> 12B
• DelayedTypeData: 40B -> 24B
• DeclarationData: 76B -> 60B
• DUContextData: 64B -> 56B
• TopDUContextData: 96B -> 80B (not instantiated *that* often)

Differential Revision: https://phabricator.kde.org/D322