Make the pretty stack trace be an opt-in, rather than opt-out, facility. Enable pretty 
stack traces by default if you use PrettyStackTraceProgram, so that existing LLVM-based 
tools will continue to get it without any changes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193971 91177308-0d34-0410-b5e6-96231b3b80d8