[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
llvm-svn: 352369