Update ORC2 API to match LLVM 21 changes
This PR replaces LLVMOrcThreadSafeContextGetContext with the new LLVMOrcCreateNewThreadSafeContextFromLLVMContext function, matching the upstream LLVM 21 C API change.
The old function was removed and replaced with one that creates a thread-safe context from an existing LLVMContext instead of extracting the inner context from a thread-safe wrapper.
This should be also backported to 22 and 21 branches, but I didn't understand how the flow works in the repo.