Fix issue 20366 - fix a programming bug in ctfe foreach_reverse code

[SSoulaimane]

Index is not decremented which causes bounds checking error.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20366	critical	CTFE foreach_reverse on array with utf characters crashes compiler

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 "stable + dmd#10547"

[thewilsonator]

please target stable

[SSoulaimane]

Sure.

[SSoulaimane]

Done.

[thewilsonator]

Auto-merge toggled on

[SSoulaimane]

Thanks.