Move pretty stack trace printer into driver.

We used to have a pretty stack trace printer in SystemInitializerCommon.
This was disabled on Apple because we didn't want the library to be
setting signal handlers, as this was causing issues when loaded into
Xcode. However, I think it's useful to have this for the LLDB driver, so
I moved it up to use the PrettyStackTraceProgram in the driver's main.

Differential revision: https://reviews.llvm.org/D49377

llvm-svn: 337261