Replace the generic zero- and one-result builders in LLVM::CallOp with a custom
builder that takes an LLVMFuncOp, which can be used to extract the result type
and create the symbol reference attribute. This is merely a convenience for
upcoming changes. The ODS-generated builders remain present.
Introduce LLVM::LLVMType::isVoidTy by analogy with the underlying LLVM type.