Fix issue 20400 - increasing array length in CTFE corrupts its value

[SSoulaimane]

Issue 20400.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20400	normal	CTFE increasing length of array of characters changes its value

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + dmd#10576"