Add an associated entity instruction and corresponding type for interface elements. (#3730)

When declaring an associated entity in an interface -- just associated
functions for now -- create an associated entity value and corresponding
type to represent a "slot in a witness table". Also track the list of
associated entities on the interface so that we will eventually be able
to check impls against them.

Associated entities are represented as the integer index of their slot
in a witness table.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/handle_function.cpp

@@ -5,6 +5,7 @@
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
 #include "toolchain/check/decl_name_stack.h"
+#include "toolchain/check/interface.h"
 #include "toolchain/check/modifiers.h"
 #include "toolchain/parse/tree_node_location_translator.h"
 #include "toolchain/sem_ir/entry_point.h"
@@ -126,9 +127,24 @@ static auto BuildFunctionDecl(Context& context,
   auto function_decl_id =
       context.AddPlaceholderInst({parse_node, function_decl});
 
+  // At interface scope, a function declaration introduces an associated
+  // function.
+  auto lookup_result_id = function_decl_id;
+  if (name_context.enclosing_scope_id_for_new_inst().is_valid() &&
+      !name_context.has_qualifiers) {
+    auto scope_inst_id = context.name_scopes().GetInstIdIfValid(
+        name_context.enclosing_scope_id_for_new_inst());
+    if (auto interface_scope =
+            context.insts().TryGetAsIfValid<SemIR::InterfaceDecl>(
+                scope_inst_id)) {
+      lookup_result_id = BuildAssociatedEntity(
+          context, interface_scope->interface_id, function_decl_id);
+    }
+  }
+
   // Check whether this is a redeclaration.
   auto existing_id =
-      context.decl_name_stack().LookupOrAddName(name_context, function_decl_id);
+      context.decl_name_stack().LookupOrAddName(name_context, lookup_result_id);
   if (existing_id.is_valid()) {
     if (auto existing_function_decl =
             context.insts().Get(existing_id).TryAs<SemIR::FunctionDecl>()) {
@@ -136,6 +152,7 @@ static auto BuildFunctionDecl(Context& context,
       function_decl.function_id = existing_function_decl->function_id;
 
       // TODO: Check that the signature matches!
+      // TODO: Disallow redeclarations within classes?
 
       // Track the signature from the definition, so that IDs in the body match
       // IDs in the signature.
@@ -149,6 +166,8 @@ static auto BuildFunctionDecl(Context& context,
       }
     } else {
       // This is a redeclaration of something other than a function.
+      // This includes the case where an associated function redeclares another
+      // associated function.
       context.DiagnoseDuplicateName(function_decl_id, existing_id);
     }
   }