Fix 21985 - Ensure that LabelDsymbols have an explicit location

[MoonlightSentinel]

This allows the error reporting to point to the first usage of an
unknown label.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21985	minor	"goto" errors with unexistent label report wrong/misleading line

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#12605"