Add tests for when a `require` decl must be satisfied before impl as (#6348)

When we `impl as Z` and `Z` is an interface with a require relationship
to another interface `Y`, we produce an error at the definition if the
self type does not impl the required interface `Y`.

The require relationship need not be satisfied yet at the declaration of
the `impl as Z`, and a declaration of `impl as Y` is enough to write the
definition of `impl as Z`.

toolchain/check/testdata/facet/require_satisified.carbon

@@ -0,0 +1,95 @@
+// 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
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/none.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/facet/require_satisified.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/facet/require_satisified.carbon
+
+// --- todo_fail_missing_require_for_concrete_type.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+interface Y {
+  extend require impls Z;
+}
+
+// () does not need to impl Z yet.
+impl () as Y;
+
+// TODO: This should be an error. () needs to impl Z at the start of the
+// definition.
+impl () as Y {}
+
+// --- todo_fail_missing_require_for_symbolic_type.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+interface Y {
+  extend require impls Z;
+}
+
+// T does not need to impl Z yet.
+impl forall [T:! type] T as Y;
+
+// TODO: This should be an error. T needs to impl Z at the start of the
+// definition.
+impl forall [T:! type] T as Y {}
+
+// --- require_for_concrete_type.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+interface Y {
+  extend require impls Z;
+}
+
+// () does not need to impl Z yet.
+impl () as Y;
+
+// Now we know () impls Z.
+impl () as Z;
+
+// So no error here.
+impl () as Y {}
+
+impl () as Z {}
+
+// --- require_for_symbolic_type.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+interface Y {
+  extend require impls Z;
+}
+
+// T does not need to impl Z yet.
+impl forall [T:! type] T as Y;
+
+// Now we know T impls Z.
+impl forall [T:! type] T as Z;
+
+// So no error here.
+impl forall [T:! type] T as Y {}
+
+impl forall [T:! type] T as Z {}
+
+// --- require_for_symbolic_type_impl_matches_same_interface.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+interface Y {
+  extend require impls Z;
+}
+
+// This only matches T that impl Z so no error here.
+impl forall [T:! Z] T as Y {}