Fix Issue 20791 - extern(C++ <strings>) should allow a trailing comma

[MoonlightSentinel]

This inconsistency was also mentioned in the spec PR

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20791	enhancement	extern(C++ ) should allow a trailing comma

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#11090"

[WalterBright]

Grammar PR for this change?

[WalterBright]

Also, do not make grammar changes to the language without my approval. Thanks.

[MoonlightSentinel]

Grammar PR for this change?

This feature isn't even documented yet because the spec PR got blocked. @Geod24 is currently investigating that issue.