Implements equalsNode for Decl and Stmt.

This is a powerful tool when doing iterative refined matches,
where another match is started inside the match callback of the first
one; this allows for example to find out whether the node was in
the condition or body of its parent if-statement.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174605 91177308-0d34-0410-b5e6-96231b3b80d8