[CFG] Add a test for a flaky crash in CFGBlock::getLastCondition().

Push the test separately ahead of time in order to find out whether
our Memory Sanitizer bots will be able to find the problem.

If not, I'll add a much more expensive test that repeats the current
test multiple times in order to show up on normal buildbots.
I really apologize for the potential temporary inconvenience!
I'll commit the fix as soon as I get the signal.

Differential Revision: https://reviews.llvm.org/D69962