Use InitLLVM to setup a pretty stack printer

InitLLVM does not only save a few lines from main() but also makes the
commands do the right thing for multibyte character pathnames on
Windows (i.e. canonicalize argv's to UTF-8) because of the code we
have in this file:

https://github.com/llvm/llvm-project/blob/master/llvm/lib/Support/InitLLVM.cpp#L32

For many LLVM commands, we already have calls of InitLLVM, but there
are still remainings.

Differential Revision: https://reviews.llvm.org/D70702