Implements hasAncestor.

Implements the hasAncestor matcher. This builds
on the previous patch that introduced DynTypedNode to build up
a parent map for an additional degree of freedom in the AST traversal.

The map is only built once we hit an hasAncestor matcher, in order
to not slow down matching for cases where this is not needed.

We could implement some speed-ups for special cases, like building up
the parent map as we go and only building up the full map if we break
out of the already visited part of the tree, but that is probably
not going to be worth it, and would make the code significantly more
complex.

Major TODOs are:
- implement hasParent
- implement type traversal
- implement memoization in hasAncestor

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163382 91177308-0d34-0410-b5e6-96231b3b80d8

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