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.

rdar://problem/27435938

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

llvm-svn: 331378