swift234

Add a check to avoid infinite looping in the pass manager.

It's possible to construct programs where the optimization pass manager
will just continually execute, never making progress.

Add a check to the pass manager that only allows us to optimize a
limited number of functions with the function passes before moving on.

Unfortunately even the tiny test case that I have for this takes minutes
before we bail out with the limit I've set (which is not *that* much
bigger than the maximum that I saw in our build). I don't think it would
be prudent to add that test to the test suite, and I haven't managed to
come up with something that finishes in a more reasonable amount of time.

rdar://problem/21260480