Improve support for qualification conversions. (#5999)

* Treat `MaybeUnformed` and `partial` as qualifiers, like `const`.
* Allow pointer conversions to add qualifiers.
* Allow unsafe pointer conversions to remove qualifiers.
* Allow conversions on non-reference expressions to drop `const`.
* Allow unsafe conversions on any expression to drop `const`.
* Allow unsafe conversions on non-initializing expressions to drop
  `partial`. For initializing expressions, we should initialize the
  vptr when dropping `partial`; this is not yet supported so we reject.
* Allow conversions on reference expressions to add `MaybeUnformed`.
* Allow unsafe conversions on reference expressions to drop
  `MaybeUnformed`. For non-reference expressions, additional work is
  required, because the value / initializing representation may not
  match between `T` and `MaybeUnformed(T)`, so those are rejected for
  now.

toolchain/testing/testdata/min_prelude/parts/maybe_unformed.carbon

@@ -0,0 +1,17 @@
+// 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/parts/destroy.carbon
+
+// --- min_prelude/parts/maybe_unformed.carbon
+
+package Core library "prelude/parts/maybe_unformed";
+
+export import library "prelude/parts/destroy";
+
+private fn Make(t: type) -> type = "maybe_unformed.make_type";
+
+class MaybeUnformed(T:! type) {
+  adapt Make(T);
+}

toolchain/testing/testdata/min_prelude/unformed.carbon

@@ -0,0 +1,15 @@
+// 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/parts/as.carbon
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/parts/maybe_unformed.carbon
+// EXTRA-ARGS: --custom-core --exclude-dump-file-prefix=min_prelude/
+
+// --- min_prelude/uint.carbon
+
+// A minimal prelude for test involving `Core.MaybeUnformed(T)`.
+package Core library "prelude";
+
+export import library "prelude/parts/as";
+export import library "prelude/parts/maybe_unformed";