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

[rodiazet]

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.

[cameel]

I think that in addition to positive tests we should also have some negative ones, showing what cannot be used as a constant.

I'd basically create a test covering all the builtins from Units and Globally Available Variables in constant assignments with literal args. Or rather a pair of tests - one for failing ones and one for passing ones, because one failure might hide errors that would happen later in analysis. This would also show if all of them behave as expected - I would not be surprised if it's not just bytes.concat() that's buggy.

[rodiazet]

Moved to #16341