Defer RequireCompleteType to impl definition (Refactor Impl construction 7/7) (#6470)

Explicitly run `RequireCompleteType` for an impl's facet type constraint
in two places:
- For a new `Impl` declaration that is `extend`
- At the start of the `Impl` definition

Stop trying to RequireCompleteType in the definition when constructing
the witness. If we have a rewrite of a name in `.Self`, then we can
construct a full witness, otherwise we defer to the definition.

Now GetOrAddImpl does not need to track `is_definition` anymore, so we
remove a lot of plumbing.

We inline the `AllocateFacetTypeImplWitness` since it has a single
caller and it is just 2 lines, to help improve understanding of the
steps and comments in setting up the impl definition.

Note that this puts the `RequreCompleteType` instruction into the
definition's generic eval block always, avoiding the issue of ensuring
that each generic redecl has the exact same instructions, and forcing
coordination to have `RequireCompleteType` inserted into every
declaration's eval block or none. The result also more closely matches
the design, with the complete type not being required until inside the
definition.

This is part of #6420 which is being split up into a chain of smaller
PRs. It is based on #6469.

toolchain/check/handle_impl.cpp

@@ -18,9 +18,11 @@
 #include "toolchain/check/pattern_match.h"
 #include "toolchain/check/type.h"
 #include "toolchain/check/type_completion.h"
+#include "toolchain/parse/node_ids.h"
 #include "toolchain/parse/typed_nodes.h"
 #include "toolchain/sem_ir/generic.h"
 #include "toolchain/sem_ir/ids.h"
+#include "toolchain/sem_ir/specific_interface.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Check {
@@ -190,8 +192,7 @@ static auto PopImplIntroducerAndParamsAsNameComponent(
 
 // Build an ImplDecl describing the signature of an impl. This handles the
 // common logic shared by impl forward declarations and impl definitions.
-static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
-                          bool is_definition)
+static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id)
     -> std::pair<SemIR::ImplId, SemIR::InstId> {
   auto [constraint_node, constraint_id] =
       context.node_stack().PopExprWithNodeId();
@@ -243,7 +244,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
          .is_final = is_final}};
     auto extend_node = introducer.modifier_node_id(ModifierOrder::Extend);
     impl_id = GetOrAddImpl(context, node_id, name.implicit_params_loc_id,
-                           impl_info, is_definition, extend_node);
+                           impl_info, extend_node);
   }
 
   // `GetOrAddImpl` either filled in the `impl_info` and returned a fresh
@@ -257,8 +258,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
 }
 
 auto HandleParseNode(Context& context, Parse::ImplDeclId node_id) -> bool {
-  auto [impl_id, impl_decl_id] =
-      BuildImplDecl(context, node_id, /*is_definition=*/false);
+  auto [impl_id, impl_decl_id] = BuildImplDecl(context, node_id);
   auto& impl = context.impls().Get(impl_id);
 
   context.decl_name_stack().PopScope();
@@ -275,8 +275,7 @@ auto HandleParseNode(Context& context, Parse::ImplDeclId node_id) -> bool {
 
 auto HandleParseNode(Context& context, Parse::ImplDefinitionStartId node_id)
     -> bool {
-  auto [impl_id, impl_decl_id] =
-      BuildImplDecl(context, node_id, /*is_definition=*/true);
+  auto [impl_id, impl_decl_id] = BuildImplDecl(context, node_id);
   auto& impl = context.impls().Get(impl_id);
 
   CARBON_CHECK(!impl.has_definition_started());
@@ -289,7 +288,6 @@ auto HandleParseNode(Context& context, Parse::ImplDefinitionStartId node_id)
       impl_decl_id, impl.scope_id,
       context.generics().GetSelfSpecific(impl.generic_id));
   StartGenericDefinition(context, impl.generic_id);
-  // This requires that the facet type is complete.
   ImplWitnessStartDefinition(context, impl);
   context.inst_block_stack().Push();
   context.node_stack().Push(node_id, impl_id);