Fix string.concat and bytes.concat assignment to a constant variable.

[rodiazet]

Initially reviewed here #16335

This PR fixes assignment to a constant variable of a result of string or bytes concatenation, if their arguments are values known in the compile-time.
Closes: #16188

Additionally

• Add unit tests
• Update the Changelog.md
• Update existing unit tests:
  • Modify tests, which trigger "no-effect" expression warning to silence the warning.
  • Update error message, because now they fail in different (but very similar) way.
  • Add more tests to cover all the builtins