[Modules] Add stdatomic to the list of builtin headers

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263076 91177308-0d34-0410-b5e6-96231b3b80d8