Generate a RequireDecl instruction for require declarations (#6318)

The `RequireDecl` instruction points, via a `RequireImplsId` to a
`RequireImpls` structure in a `ValueStore`. That structure holds the
self-type and facet type, as well as the generic id and parent scope.
`RequireImpls` is always a generic since it only appears in an
`interface` or `constraint`, which both have a generic parameter `Self`
applied to all their members.

The `RequireDecl` instruction evaluates to itself, but drops the
decl_block_id since the instructions within the `require` declaration
are not required in the canonical value which is only used for import.
And import will want to import the `RequireImpls` structure along with
the `Interface` or `NamedConstraint` structure it is in, rather than
recreate it from the decl's instructions. This also avoids repeating all
the instructions within the `require` decl in the textual semir's
constants block.

Adding the `RequireImpls` to the `Interface` or `NamedConstraint`
structure is not yet done, so they are not available for impl lookup or
import yet.

toolchain/check/handle_impl.cpp

@@ -190,6 +190,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
                                /*is_extern=*/false, SemIR::LibraryNameId::None),
                            {.self_id = self_type_inst_id,
                             .constraint_id = constraint_type_inst_id,
+                            // This requires that the facet type is identified.
                             .interface = CheckConstraintIsInterface(
                                 context, impl_decl_id, constraint_type_inst_id),
                             .is_final = is_final}};
@@ -230,6 +231,7 @@ auto HandleParseNode(Context& context, Parse::ImplDefinitionStartId node_id)
       impl_decl_id, impl_info.scope_id,
       context.generics().GetSelfSpecific(impl_info.generic_id));
   StartGenericDefinition(context, impl_info.generic_id);
+  // This requires that the facet type is complete.
   ImplWitnessStartDefinition(context, impl_info);
   context.inst_block_stack().Push();
   context.node_stack().Push(node_id, impl_id);