[Support] Use O_CLOEXEC only when declared

Summary:
Use the O_CLOEXEC flag only when it is available. Some old systems (e.g.
SLES10) do not support this flag. POSIX explicitly guarantees that this
flag can be checked for using #if, so there is no need for a CMake
check.

In case O_CLOEXEC is not supported, fall back to fcntl(FD_CLOEXEC)
instead.

Reviewers: rnk, rafael, mgorny

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28894

llvm-svn: 292912