[IRGenDebugInfo] Disable usage of dbg.declare in optimized code
Except for the async context, where it is needed (arguments
within an async function).
We don't support dbg.declare in optimized code, as variables can
be moved by SIL optimization passes. If a partial store is
eliminated, we want a dbg.value on the allocation, and another
dbg.value with a fragment in place of the partial store.
rdar://128155050
