[llvm] remove references to hard-coded gc strategies (PR #94401)

[Dwight Guth via llvm-commits]

dwightguth wrote:

If someone could recommend how I might be able to write a test, that might be helpful. I admit I don't fully understand, because these changes only really affect non built-in GC strategies and I don't fully understand how I can write a test that makes use of a custom GCStrategy object.

https://github.com/llvm/llvm-project/pull/94401