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/Host/posix/FileSystem.cpp b/lldb/source/Host/posix/FileSystem.cpp
index b974c44..1f2e7db 100644
--- a/lldb/source/Host/posix/FileSystem.cpp
+++ b/lldb/source/Host/posix/FileSystem.cpp
@@ -32,6 +32,9 @@
 using namespace lldb;
 using namespace lldb_private;
 
+const char *
+FileSystem::DEV_NULL = "/dev/null";
+
 FileSpec::PathSyntax
 FileSystem::GetNativePathSyntax()
 {