Revert "Follow-up to r331378. Update tests to allow to use C atomics in C++."

It reverts commit r331484 because it caused test failures

    ThreadSanitizer-x86_64 :: Darwin/gcd-groups-destructor.mm
    ThreadSanitizer-x86_64 :: Darwin/libcxx-shared-ptr-stress.mm
    ThreadSanitizer-x86_64 :: Darwin/xpc-race.mm

Foundation.h transitively includes <atomic>, so we have a case of benign mixing
<stdatomic.h> and <atomic>.

llvm-svn: 331491