Revert "Refactor mangle-only extern(C++, namespace) into an AttribDeclaration"

[WalterBright]

Reverts #10021 which no longer has a purpose since its follow-on #10031 is not going to happen.

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

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#10037"

[Geod24]

What ? Read the PR again. #10031 was only mentioned in the context of test coverage, not as a motivation for the PR.

The point of #10021 was simple: extern(C++, "namespace") created a ScopeDsymbol then worked around the scope-introducing behavior. That was very unnatural, unscalable, and error prone. Now extern(C++, "namespace") is an AttribDeclaration and the only special handling it requires is in cppmangle.

[WalterBright]

I looked at it again, sorry, you're right.