Fixes missing source locations for ReturnStatement nodes in generated
ast. Simple change using existing pattern, only required changes to the
codegen step, no other pipeline changes.

**Most file changes are new lines in generated code.** [First
commit](d15e90e)
has the relevant changes, second commit has the noisy snap updates.

I added an exception to the validator to not report an error when a
return statement will be optimized to an implicit return by codegen, as
there's no separate return statement to instrument anyways in the final
ast. An edge case when it comes to preserving source locations for
instrumentation that is likely not as common for most babel transforms
since they are not doing optimizations.