Make test/CodeGen/atomic-ops.c free-standing

This test includes stdint.h (via stdatomic.h), which might include system
headers (and that might not work, depending on the system configuration).
Attempting to fix llvm-clang-lld-x86_64-debian-fast.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218960 91177308-0d34-0410-b5e6-96231b3b80d8