Fix crash on accessing function parameters

We're currently hitting a segfault when trying to iterate over the arguments of a newly-constructed function. Here's a piece of the traceback, from LLDB:

```
* frame #0: 0x0000000106116f88 libLLVM.dylib`llvm::LLVMContext::shouldDiscardValueNames() const + 4
frame #1: 0x0000000106145b77 libLLVM.dylib`llvm::Value::setNameImpl(llvm::Twine const&) + 39
frame #2: 0x0000000106145e0c libLLVM.dylib`llvm::Value::setName(llvm::Twine const&) + 14
frame #3: 0x00000001060f5f7a libLLVM.dylib`llvm::Argument::Argument(llvm::Type*, llvm::Twine const&, llvm::Function*) + 110
frame #4: 0x00000001060f702b libLLVM.dylib`llvm::Function::BuildLazyArguments() const + 105
frame #5: 0x00000001060d9dd1 libLLVM.dylib`LLVMGetParam + 27
```