Sema: Permit an atomic type to be initialized by the same atomic type

We forgot a conversion step when initializing an atomic type with an
rvalue of the same type.

This fixes PR22043.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224902 91177308-0d34-0410-b5e6-96231b3b80d8