Add support for bitwise operators. (#1809)

Following #1191, add initial support for bitwise operators. Support for both
integer operations and for operator overloading via the interfaces specified in
#1191 is provided.

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

explorer/testdata/operators/bit_complement.carbon

@@ -0,0 +1,24 @@
+// 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
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: -6
+
+package ExplorerTest api;
+
+class A { var n: i32; }
+
+external impl A as BitComplement where .Result == A {
+  fn Op[me: Self]() -> A { return {.n = ^me.n}; }
+}
+
+fn Main() -> i32 {
+  var a: A = {.n = 5};
+  a = ^a;
+  return a.n;
+}