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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154612 91177308-0d34-0410-b5e6-96231b3b80d8