adding missing exit for pattern variable in wrong context and a test case for it (#324)

executable_semantics/testdata/pattern_variable_fail.6c

@@ -0,0 +1,8 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn main() -> Int {
+  Int : x;  // error
+  return 1;
+}