Use constant evaluation to determine the identity of types. (#3617)

Remove the type canonicalization mechanism and instead rely on constant
canonicalization to deduplicate types.

Rename the `Canonicalize*Type` functions to reflect that they're no
longer performing canonicalization. Switch code that creates types due
to semantic checking, rather than due to source syntax, to directly
create type constants through evaluation rather than creating an
instruction and evaluating it to produce a separate constant
representation.

The mapping from `const (const T)` that was previously performed by type
canonicalization is now implemented in expression evaluation instead.

The value `<error>` is now treated as a constant value, with a special
property that an instruction involving `<error>` that could possibly be
constant evaluates to `<error>`. This helps avoid producing follow-on
errors when an error occurs as a subexpression of an expression, such as
a type, that is intended to be constant.

toolchain/check/handle_function.cpp

@@ -191,7 +191,7 @@ static auto BuildFunctionDecl(Context& context,
         (return_slot_id.is_valid() &&
          return_type_id !=
              context.GetBuiltinType(SemIR::BuiltinKind::BoolType) &&
-         return_type_id != context.CanonicalizeTupleType({}))) {
+         return_type_id != context.GetTupleType({}))) {
       CARBON_DIAGNOSTIC(InvalidMainRunSignature, Error,
                         "Invalid signature for `Main.Run` function. Expected "
                         "`fn ()` or `fn () -> i32`.");