[clang-diff] Simplify mapping

Summary:
Until we find a decent heuristic on how to choose between multiple
identical trees, there is no point in supporting multiple mappings.

This also enables matching of nodes with parents of different types,
because there are many instances where this is appropriate.  For
example for and foreach statements; functions in the global or
other namespaces.

Reviewers: arphaman

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D36183

llvm-svn: 311251