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

[mgorny]

Summary

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));
                                                     ^~~~~~~~~

llvm/trunk/lib/Support/Unix/Signals.inc

for (int i = 0; i < depth; ++i) {
     if (dlinfo.dli_sname != nullptr) {
       OS << ' ';
       int res;
       char* d = itaniumDemangle(dlinfo.dli_sname, nullptr, nullptr, &res);
       if (!d) OS << dlinfo.dli_sname;
       else    OS << d;
       free(d);
 
-      // FIXME: When we move to C++11, use %t length modifier. It's not in
-      // C++03 and causes gcc to issue warnings. Losing the upper 32 bits of
-      // the stack offset for a stack dump isn't likely to cause any problems.
-      OS << format(" + %u",(unsigned)((char*)StackTrace[i]-
-                                      (char*)dlinfo.dli_saddr));
+      OS << format(" + %tu", (static_cast<const char*>(StackTrace[i])-
+                              static_cast<const char*>(dlinfo.dli_saddr)));
     }
     OS << '\n';
   }
 #elif defined(HAVE_BACKTRACE)
   backtrace_symbols_fd(StackTrace, depth, STDERR_FILENO);
 #endif
 #endif
 }