[llvm] [Support] Clean PrintStackTrace() ptr arithmetic up

Use '%tu' modifier for pointer arithmetic since we are using C++11
already.  Prefer static_cast<> over C-style cast.  Remove unnecessary
conversion of result, and add const qualifier to converted pointers,
to silence the following warning:

  In file included from /home/mgorny/llvm-project/llvm/lib/Support/Signals.cpp:220:0:
  /home/mgorny/llvm-project/llvm/lib/Support/Unix/Signals.inc: In function ‘void llvm::sys::PrintStackTrace(llvm::raw_ostream&)’:
  /home/mgorny/llvm-project/llvm/lib/Support/Unix/Signals.inc:546:53: warning: cast from type ‘const void*’ to type ‘char*’ casts away qualifiers [-Wcast-qual]
                                         (char*)dlinfo.dli_saddr));
                                                       ^~~~~~~~~

Differential Revision: https://reviews.llvm.org/D63888

llvm-svn: 364912