[AST] AST structural equivalence to work internally with pairs.

[AST] AST structural equivalence to work internally with pairs.

Summary:
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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66538

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370639 91177308-0d34-0410-b5e6-96231b3b80d8