Add support for _ placeholder. (#661)

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/testdata/ignored_parameter.carbon

@@ -0,0 +1,11 @@
+// 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 ReturnSecond(_: Int, x: Int) -> Int {
+  return x;
+}
+
+fn main() -> Int {
+  return ReturnSecond(1, 0);
+}

executable_semantics/testdata/ignored_parameter.golden

@@ -0,0 +1 @@
+result: 0

executable_semantics/testdata/match_placeholder.carbon

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+// 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 {
+  var t: auto = (1, 2, 3, 4);
+  match (t) {
+    case (_: Int, _: auto, x: Int, y: auto) =>
+      return y - x - 1;
+  }
+}

executable_semantics/testdata/match_placeholder.golden

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+result: 0

executable_semantics/testdata/placeholder_variable.carbon

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+// 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 {
+  var _: auto = 1;
+  return 0;
+}

executable_semantics/testdata/placeholder_variable.golden

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+result: 0