Atomics: support __c11_* calls on _Atomic struct types.

When a struct's size is not a power of 2, the corresponding _Atomic() type is
promoted to the nearest. We already correctly handled normal C++ expressions of
this form, but direct calls to the __c11_atomic_whatever builtins ended up
performing dodgy operations on the smaller non-atomic types (e.g. memcpy too
much). Later optimisations removed this as undefined behaviour.

This patch converts EmitAtomicExpr to allocate its temporaries at the full
atomic width, sidestepping the issue.

llvm-svn: 252507