pub fn get_value(&self, sym: &Symbol) -> WeldResult<LLVMValueRef> {
        self.symbols.get(sym).cloned().ok_or_else(|| {
            WeldCompileError::new(format!("Undefined symbol {} in function codegen", sym))
        })
    }
Returns the LLVM value for a symbol in this function.
weld/src/codegen/llvm2/mod.rs