Carbon crash on code snippet with redefined variable

### Description of the bug: Crashed when I accidentally duplicated a line and ended up defining a variable twice, once bound and once unformed. ### What did you do, or what's a simple way to reproduce the bug? https://carbon.compiler-explorer.com/z/5PaTTeacs ### What did you expect to happen? Warn me I was redefining my variable ### What actually happened? Compiler crash ### Any other information, logs, or outputs that you want to share? ``` package Test; // Starting new example file // Carbon // A quick class for discussion class AClassType { fn Make() -> AClassType { return {}; } }; var some_object_defined_earlier: AClassType = AClassType.Make(); var some_object_defined_earlier = AClassType; ```