Fix a problem where we were not calling fcntl() with the correct arguments for F_DUPFD

On Mac OS X, this was working just fine in debug builds (presumably, because the right value ended up being at the right location for the variadic ABI), but not in Release builds
As a result, we were seeing failures with commands that set their own immediate output stream - only in Release builds, which always makes for a fun little investigation

I have removed those fcntl() calls and replaced them with dup() calls. This fixes the issue in both Debug and Release builds

llvm-svn: 258367