[ASTMatchers] Do not try to memoize nodes we can't compare.

Summary:
Prevent hasAncestor from comparing nodes that are not supported.
hasDescendant was fixed some time ago to avoid this problem.
I'm applying the same fix to hasAncestor: if any object in the Builder map is
not comparable, skip the cache.

Reviewers: alexfh

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D19231

llvm-svn: 266748