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

It reverts commit r331379 because turned out `__ALLOW_STDC_ATOMICS_IN_CXX__`
doesn't work well in practice.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@331818 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed