Initial support for where expressions. (#1310)

Build constraint types from `where` expressions. This is very bare-bones; there's no support for constraining `.Self` or associated types yet, but degenerate cases not involving self-reference work.

`==` constraints are syntactically supported but not semantically verified yet, and they aren't used for anything.

explorer/interpreter/interpreter.cpp

@@ -412,6 +412,7 @@ auto Interpreter::StepLvalue() -> ErrorOr<Success> {
     case ExpressionKind::ValueLiteral:
     case ExpressionKind::IntrinsicExpression:
     case ExpressionKind::IfExpression:
+    case ExpressionKind::WhereExpression:
     case ExpressionKind::ArrayTypeLiteral:
     case ExpressionKind::InstantiateImpl:
       CARBON_FATAL() << "Can't treat expression as lvalue: " << exp;
@@ -1082,6 +1083,10 @@ auto Interpreter::StepExp() -> ErrorOr<Success> {
       }
       break;
     }
+    case ExpressionKind::WhereExpression: {
+      return todo_.FinishAction(
+          &cast<TypeOfConstraintType>(exp.static_type()).constraint_type());
+    }
     case ExpressionKind::UnimplementedExpression:
       CARBON_FATAL() << "Unimplemented: " << exp;
     case ExpressionKind::ArrayTypeLiteral: {