Fix issue 14178 - extern(C++) doesn't mangle 'std' correctly on posix systems

[TurkeyMan]

No description provided.

[dlang-bot]

Thanks for your pull request and interest in making D better, @TurkeyMan! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18957	normal	extern(C++) doesn't mangle 'std' correctly on posix systems

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#8341"

[Geod24]

Thanks. Updated the title to reference https://issues.dlang.org/show_bug.cgi?id=14178