Support explicit conversion of tuples. (#2537)

Allow explicit conversion from a tuple of `T1`, `T2`, ... to a tuple of `U1`, `U2`, ... if each element has an explicit conversion.

Also add a comment to existing `ImplicitAs` impl for tuples explaining its purpose.

explorer/testdata/tuple/convert.carbon

@@ -0,0 +1,42 @@
+// 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
+//
+// AUTOUPDATE
+// RUN: %{explorer-run}
+// RUN: %{explorer-run-trace}
+// CHECK:STDOUT: 2
+// CHECK:STDOUT: 3
+// CHECK:STDOUT: 4
+// CHECK:STDOUT: 5
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+class A {
+  var a: i32;
+}
+class B {
+  var b: i32;
+}
+class C {
+  var c: i32;
+}
+
+external impl A as ImplicitAs(B) {
+  fn Convert[self: Self]() -> B { return {.b = self.a + 1}; }
+}
+external impl B as As(C) {
+  fn Convert[self: Self]() -> C { return {.c = self.b + 2}; }
+}
+
+fn Main() -> i32 {
+  var a: (A, A) = ({.a = 1}, {.a = 2});
+  var b: (B, B) = a;
+  Print("{0}", b[0].b);
+  Print("{0}", b[1].b);
+  var c: (C, C) = b as (C, C);
+  Print("{0}", c[0].c);
+  Print("{0}", c[1].c);
+  return 0;
+}

explorer/testdata/tuple/fail_implicit_convert_with_as.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
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+class A {
+  var a: i32;
+}
+class B {
+  var b: i32;
+}
+
+external impl A as As(B) {
+  fn Convert[self: Self]() -> B { return {.b = self.a}; }
+}
+
+fn Main() -> i32 {
+  var a: (i32, A) = (1, {.a = 2});
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_implicit_convert_with_as.carbon:[[@LINE+1]]: type error in name binding: '(i32, class A)' is not implicitly convertible to '(i32, class B)'
+  var b: (i32, B) = a;
+  return 0;
+}