Unify XDEBUG and EXPENSIVE_CHECKS (into the latter), and add an option to the cmake build to enable them.

Summary:
Historically, we had a switch in the Makefiles for turning on "expensive
checks". This has never been ported to the cmake build, but the
(dead-ish) code is still around.

This will also make it easier to turn it on in buildbots.

Reviewers: chandlerc

Subscribers: jyknight, mzolotukhin, RKSimon, gberry, llvm-commits

Differential Revision: http://reviews.llvm.org/D19723

llvm-svn: 268050