Fix task macro's handling of Symbol owners in <qual>.value

The qualifier of the `.value` call may contain `DefTree`s (e.g.
vals, defs) or `Function` trees. When we snip them out of the
tree and graft them into a new context, we must also call
`changeOwner`, so that the symbol owner structure and the tree
structure are coherent.

Failure to do so resulted in a crash in the compiler backend.

Fixes #1150

Review by @jsuereth / @gkossakowski