Fix 21634 - std.bitmanip: bitfields may generate invalid variable

[berni44]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21634	minor	std.bitmanip: bitfields may generate invalid variable

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 + phobos#7799"