[Modules] Add stdatomic to the list of builtin headers

Since it's provided by the compiler. This allows a system module map
file to declare a module for it.

No test change for cstd.m, since stdatomic.h doesn't function without a
relatively complete stdint.h and stddef.h, which tests using this module
don't provide.

rdar://problem/24931246

llvm-svn: 263076