[LLDB] [lldb-server] Use llvm::InitLLVM for doing unicode conversion of arguments for windows

This should allow lldb-server to operate on files with non-ascii
pathnames.

I tried looking around in lldb/tools, and this seemed like the only
other tool (other than the main lldb driver itself) that would be
used (implicitly) by an end user (which could be working in
non-ascii paths).

Differential Revision: https://reviews.llvm.org/D68864

llvm-svn: 374537