Fix some i1/i8 confusion within _Atomic(bool) in IR generation, both

in general (such an atomic has boolean representation) and
specifically for IR generation of __c11_atomic_init. The latter also
means actually using initialization semantics for this initialization,
rather than just creating a store.

On a related note, make sure we actually put in non-atomic-to-atomic
conversions when performing an implicit conversion sequence. IR
generation is far too kind here, but we still want the ASTs to make
sense.

llvm-svn: 154612