Move the internal PrintStackTrace function that is used for llvm::sys::PrintStackTraceOnErrorSignal(),
into a new function llvm::sys::PrintStackTrace, so that it's available to clients for logging purposes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171989 91177308-0d34-0410-b5e6-96231b3b80d8