Make type contexts expect a value of type Type. (#2357)

In particular, this means that a type can implement `ImplicitAs(Type)` and have values of that type behave like types.

This implies that `()` and `{}` are no longer types. They are now values whose type is the result of converting `()` or `{}` to type `Type`, as has been discussed recently and seems to be the supported direction. This fixes various cases where these types were previously mishandled.

explorer/testdata/array/element_convertible_to_type.carbon

@@ -0,0 +1,22 @@
+// 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: 3
+
+package ExplorerTest api;
+
+class TypeLike {
+  impl as ImplicitAs(Type) {
+    fn Convert[me: Self]() -> Type { return i32; }
+  }
+  fn Make() -> Self { return {}; }
+}
+
+fn Main() -> TypeLike.Make() {
+  var v: [TypeLike.Make(); 2] = (1, 2);
+  return v[0] + v[1];
+}