The question was "What interpreters did these orgs write in GCed languages?".
It doesn't matter where those interpreters are being used, the fact is that they exist.
> Microsoft's C# CoreCLR is C++.
Yes, but they have moved their compilers to C# with the Rosylin project with MDIL and .NET Native moving more code to C# side.
Also check the Phoenix compiler framework, a LLVM like project from MSR using C#.
> Go's core runtime is in a Go-minus-GC subset.
Of course, there needs to exist a Genesis for the GC, but once it is there, 99% of the remaining infrastructure makes use of it.
If you take Nashorn engine running on Jikes, it is an 100% Java stack running a JavaScript interpreter.
> For whatever reason, when it counts, the stuff under the hood doesn't use garbage collection.
Most of the time it is just building on top of whatever already exists instead of starting from scratch.
Taking Swift as an example bootstraping the language would increase the burden designing it, as they are still far from fully stable design.
If they would throw away in man years invested into the LLVM, which happens to be written in C++, it would take ages before pure Swift compilers would ever manage to catch up.
However these shortcuts open the door for those not savvy in compiler design to use as ammunition against secure languages.