fix Bugzilla 23803 - compile-time error for concatenation in -betterC

[royalpinto007]

Fixes an issue where string/array concatenation (~) in -betterC mode caused a linker error instead of a compile-time error.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23803	regression	Link error instead of error message when using betterC

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

To target stable perform these two steps:

1. Rebase your branch to upstream/stable:

git rebase --onto upstream/stable upstream/master

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#17492"

[dkorpel]

This doesn't fix the bug, it just changes the wording of the error message and adds a modified version of the test case where the bug doesn't trigger. (foo() is no longer a template / auto return function)