Following r297661, disable dup workaround to disable duplicate STDOUT fd...

Following r297661, disable dup workaround to disable duplicate STDOUT fd closing and instead directly prevent closing of STD* file descriptors. 

We do not want to close STDOUT as there may have been several uses of it
such as the case: llc %s -o=- -pass-remarks-output=- -filetype=asm
which cause multiple closes of STDOUT_FILENO and/or use-after-close of it.
Using dup() in getFD doesn't work as we end up with original STDOUT_FILENO
open anyhow.

reviewed by Rafael Espindola

Differential Revision: https://reviews.llvm.org/D31505

llvm-svn: 299098