Add boolean to PrintStackTraceOnErrorSignal to disable crash reporting.

The current crash reporting on Mac OS is only disabled via an environment variable.
This adds a boolean (default false) which can also disable crash reporting.

The only client right now is the unittests which don't ever want crash reporting, but do want to detect killed programs.

Reduces the time to run the APFloat unittests on my machine from

[----------] 47 tests from APFloatTest (51250 ms total)

to

[----------] 47 tests from APFloatTest (765 ms total)

Reviewed by Reid Kleckner and Justin Bogner

llvm-svn: 234353