Remove ExpectType and some calls to IsImplicitlyConvertible. (#2647)

These functions are dangerous, as they check whether conversions are possible without actually performing the conversions. In each case where they were used, explorer would crash in some cases if a user-defined conversion is required.

This change moves us more towards implicit conversions being handled by a regular function call on an interface and away from them being magical builtins. Unfortunately, this exposes a pre-existing bug that a call of the form `x.(ImplicitAs(T).Convert)()` compiles even if `x` only has an explicit conversion to `T`. That's worked around here for now, but will need a proper fix later.

explorer/testdata/pointer/fail_invalid_ptr_conversion1.carbon

@@ -13,7 +13,7 @@ class A {}
 fn Main() -> i32 {
   var a: A = {};
   var b: A* = &a;
-  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/pointer/fail_invalid_ptr_conversion1.carbon:[[@LINE+1]]: type error in name binding: 'class A*' is not implicitly convertible to 'i32'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/pointer/fail_invalid_ptr_conversion1.carbon:[[@LINE+1]]: type error in initializer of variable: 'class A*' is not implicitly convertible to 'i32'
   var c: i32 = b;
   return 1;
 }

explorer/testdata/pointer/fail_invalid_ptr_conversion2.carbon

@@ -13,7 +13,7 @@ class A {}
 fn Main() -> i32 {
   var a: i32 = 0;
   var b: i32* = &a;
-  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/pointer/fail_invalid_ptr_conversion2.carbon:[[@LINE+1]]: type error in name binding: 'i32*' is not implicitly convertible to 'class A*'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/pointer/fail_invalid_ptr_conversion2.carbon:[[@LINE+1]]: type error in initializer of variable: 'i32*' is not implicitly convertible to 'class A*'
   var c: A* = b;
   return 1;
 }

explorer/testdata/pointer/pointer_to_type_like.carbon

@@ -0,0 +1,25 @@
+// 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: result: 1
+
+package ExplorerTest api;
+
+class TypeLike {
+  var v: type;
+  impl as ImplicitAs(type) {
+    fn Convert[self: Self]() -> type { return i32; }
+  }
+}
+
+fn Almosti32() -> TypeLike { return {.v = i32}; }
+
+fn Main() -> i32 {
+  var a: Almosti32() = 1;
+  var p: Almosti32()* = &a;
+  return *p;
+}

explorer/testdata/pointer/tuple_pointer.carbon

@@ -0,0 +1,25 @@
+// 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: 3
+// CHECK:STDOUT: 4
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var a: (i32, i32) = (1, 2);
+  var p: (i32, i32)* = &a;
+
+  a[0] = 3;
+  Print("{0}", (*p)[0]);
+
+  (*p)[1] = 4;
+  Print("{0}", a[1]);
+
+  return 0;
+}