Fix: const_eval bugs

[xunilrj]

Description

This PR is necessary for #4708

It fixes some problems with const_eval, most importantly:

1 - It differentiates when an eval cannot be used inside const (there is a new error for that), and when expressions/statements just do not produce any value;
2 - It always cleans up eval_codeblock (it never shortcircuit with ?);

Another advantage of the explicit error is the exact span of the "offending" expression, which we can use in the future for better error messages.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[anton-trunov]

It would be great to have failing end-to-end tests with e.g. return and state-modifying operators somewhere in this directory: https://github.com/FuelLabs/sway/tree/master/test/src/e2e_vm_tests/test_programs/should_fail

[xunilrj]

It would be great to have failing end-to-end tests with e.g. return and state-modifying operators somewhere in this directory: https://github.com/FuelLabs/sway/tree/master/test/src/e2e_vm_tests/test_programs/should_fail

Done.