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;
+}

executable_semantics/testdata/pattern_variable_fail.golden

@@ -0,0 +1,11 @@
+********** source program **********
+fn main () -> Int {
+Int: x;
+return 1;
+
+}
+********** type checking **********
+--- step exp Int --->
+--- step exp Int --->
+2: compilation error, pattern variables are only allowed in pattern context
+EXIT CODE: 255