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/basic_syntax/choice.carbon

@@ -22,7 +22,7 @@ fn Main() -> i32 {
   var y: Ints = Ints.One(42);
   var n: i32 = 0;
   match (y) {
-    case Ints.None =>
+    case Ints.None() =>
       n = n + 2;
     case Ints.One(x: auto) =>
       n = x + 1 - 42;

explorer/testdata/basic_syntax/fail_choice_no_parens.carbon

@@ -0,0 +1,27 @@
+// 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: %{not} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+
+package ExplorerTest api;
+
+choice Ints {
+  None,
+  One(i32),
+  Two(i32, i32)
+}
+
+fn Main() -> i32 {
+  match (Ints.None()) {
+    case Ints.None => return 0;
+    // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_choice_no_parens.carbon:[[@LINE+3]]: type error in `match` pattern type
+    // CHECK: expected: fn () -> choice Ints
+    // CHECK: actual: choice Ints
+    default => return 1;
+  }
+}