Identify and complete facet types as needed for p5168 (#6369)

Proposal #5168 defines when a facet type must be identified or complete,
and what it means for an interface and a named constraint to be
identified or complete. This updates the toolchain to match the
requirements.

This implements identification of a facet type to require completed
named constraints and to include any interfaces from named constraints
into the resulting IdentifiedFacetType.

To complete a facet type, each interface in the IdentifiedFacetType, and
any interface named though a require declaration from them, must be
complete.

toolchain/check/impl.cpp

@@ -121,6 +121,7 @@ auto ImplWitnessStartDefinition(Context& context, SemIR::Impl& impl) -> void {
       witness_block.empty()) {
     if (!RequireCompleteFacetTypeForImplDefinition(
             context, SemIR::LocId(impl.latest_decl_id()), impl.constraint_id)) {
+      FillImplWitnessWithErrors(context, impl);
       return;
     }