FIX Issue 19643 - add missing Objective-C function calling convention

[kubo39]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19643	minor	core.demangle doesn't handle extern(Objective-C) correctly

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 + druntime#2482"

[thewilsonator]

LGTM, cc @jacob-carlborg

[rainers]

The mangling for Objective-C has an ambiguity if used with the grammar from the website, see https://issues.dlang.org/show_bug.cgi?id=17310. IIRC the demangler should be able to handle these nevertheless, but you might want to add some symbols from that bug report to the tests, too.