C11 defines `kill_dependency` as a macro in <stdatomic.h>. When you
include <atomic> after <stdatomic.h>, the macro clashes with
`std::kill_dependency` and causes multiple errors. Explicit error should
help in diagnosing those errors.
No change for working code that includes <atomic> before <stdatomic.h>.
rdar://problem/27435938
Reviewers: rsmith, EricWF, mclow.lists, jfb
Reviewed By: jfb
Subscribers: jfb, jkorous-apple, christof, bumblebritches57, JonChesterfield, smeenai, cfe-commits
Differential Revision: https://reviews.llvm.org/D45470
llvm-svn: 332413