Remove Host::Backtrace in favor of llvm::sys::PrintStackTrace()

This removes Host::Backtrace from the codebase, and changes all
call sites to use llvm::sys::PrintStackTrace().  This makes the
functionality available for all platforms, and even for platforms
which currently had a supported implementation of Host::Backtrace,
this patch should enable richer information in stack traces, such
as file and line number information, as well as giving it the
ability to unwind through inlined functions.

llvm-svn: 231511