Revert "Fix `impl_trait` function to emit correct ast"
This reverts commit 55a4813151a8f36dcdb520c45a461fe5dfbed499.
Fix `impl_def_from_trait`
It now generates the correct `ast::Impl` using
`generate_trait_impl_text` and parses it to form the right node (copied
from the private fn 'make::ast_from_text').