[AST] Introduce GenericSelectionExpr::Association
Introduce a new class GenericSelectionExpr::Association which bundle together
an association expression and its TypeSourceInfo.
An iterator GenericSelectionExpr::AssociationIterator is additionally added to
make it possible to iterate over ranges of Associations. This iterator is a
kind of proxy iterator which abstract over how exactly the expressions and the
TypeSourceInfos are stored.
Differential Revision: https://reviews.llvm.org/D57106
Reviewed By: aaron.ballman
Reviewers: aaron.ballman, steveire, dblaikie, mclow.lists