Make uses of /dev/null portable across OSes.

Most platforms have "/dev/null".  Windows has "nul".  Instead of
hardcoding the string /dev/null at various places, make a constant
that contains the correct value depending on the platform, and use
that everywhere instead.

llvm-svn: 250331
diff --git a/lldb/source/Target/Process.cpp b/lldb/source/Target/Process.cpp
index cf30906..e634c1d 100644
--- a/lldb/source/Target/Process.cpp
+++ b/lldb/source/Target/Process.cpp
@@ -21,6 +21,7 @@
 #include "lldb/Expression/UserExpression.h"
 #include "lldb/Expression/IRDynamicChecks.h"
 #include "lldb/Host/ConnectionFileDescriptor.h"
+#include "lldb/Host/FileSystem.h"
 #include "lldb/Host/Host.h"
 #include "lldb/Host/HostInfo.h"
 #include "lldb/Host/Pipe.h"
@@ -478,7 +479,7 @@
         case 'n':   // Disable STDIO
         {
             FileAction action;
-            const FileSpec dev_null{"/dev/null", false};
+            const FileSpec dev_null{FileSystem::DEV_NULL, false};
             if (action.Open(STDIN_FILENO, dev_null, true, false))
                 launch_info.AppendFileAction (action);
             if (action.Open(STDOUT_FILENO, dev_null, false, true))