First step towards adding a parent map to the ASTContext.

This does not yet implement the LimitNode approach discussed.

The impact of this is an O(n) in the number of nodes in the AST
reduction of complexity for certain kinds of matchers (as otherwise the
parent map gets recreated for every new MatchFinder).

See FIXMEs in the comments for the direction of future work.

llvm-svn: 176251