Add a file open flag that disables O_CLOEXEC.

O_CLOEXEC is the right default, but occasionally you don't
want this.  This is especially true for tools like debuggers
where you might need to spawn the child process with specific
files already open, but it's occasionally useful in other
scenarios as well, like when you want to do some IPC between
parent and child.

llvm-svn: 334293