Revert "Emit an error when mixing <stdatomic.h> and <atomic>"

It reverts r331378 as 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

Only clang part of the change is reverted, libc++ part remains as is because it
emits error less aggressively.

llvm-svn: 331392