Support user-defined implicit conversions via ImplicitAs (#1273)

Support is added for all of the non-type expression contexts where we currently accept built-in conversions, such as reordering the fields in a struct.

explorer/testdata/while/convert_condition.carbon

@@ -0,0 +1,33 @@
+// 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: 0
+
+package ExplorerTest api;
+
+class LazyNe {
+  var v1: i32;
+  var v2: i32;
+  impl as ImplicitAs(Bool) {
+    fn Convert[me: Self]() -> Bool {
+      return not (me.v1 == me.v2);
+    }
+  }
+  fn Make(v1: i32, v2: i32) -> Self {
+    return {.v1 = v1, .v2 = v2};
+  }
+}
+
+fn Main() -> i32 {
+  var x: auto = 2;
+  while (LazyNe.Make(x, 0)) {
+    x = x - 1;
+  }
+  return x;
+}