[AST] AST structural equivalence to work internally with pairs.
The structural equivalence check stores now pairs of nodes in the
'from' and 'to' context instead of only the node in 'from' context
and a corresponding one in 'to' context. This is needed to handle
cases when a Decl in the 'from' context is to be compared with
multiple Decls in the 'to' context.
Reviewers: martong, a_sidorin
Reviewed By: martong, a_sidorin
Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits
Differential Revision: https://reviews.llvm.org/D66538