Whenever we visit a declaration via the DeclChecker, add it to the
list of declarations to finalize. This makes sure that we can centralize
the notion of “finalize for SILGen” and that it will be called for
everything in the source file being processed.
We saw manual migration is necessary on these cases to add 'as Type'. This patch starts
to issue compiler fixits on return statement and initialization just like in other type-mismatch
cases.
