Emit an error when mixing <stdatomic.h> and <atomic>
Atomics in C and C++ are incompatible at the moment and mixing the
headers can result in confusing error messages.
Emit an error explicitly telling about the incompatibility. Introduce
the macro __ALLOW_STDC_ATOMICS_IN_CXX__ that allows to choose in C++
between C atomics and C++ atomics.
Reviewers: rsmith, EricWF, mclow.lists
Reviewed By: mclow.lists
Subscribers: jkorous-apple, christof, bumblebritches57, JonChesterfield, smeenai, cfe-commits
Differential Revision: https://reviews.llvm.org/D45470