Move forward-decl-only code out to the handler of the forward decl node (Refactor Impl construction 3/7)

[danakj]

Rather than run the code for both decl and defn and make it conditional on not being a definition, put the code in the handler for the Parse::ImplDeclId node, which is handled when there's no definition. This will help lead us to no longer needing to plumb around is_definition later.

Make some naming consistent to call the reference to an Impl as impl instead of sometimes impl_info.

This is part of #6420 which is being split up into a chain of smaller PRs. It is based on #6466.