Pass imports to SemIR. (#3415)

This adds instructions so that we get printing. I may adjust the
instruction format a little further to add a type, but I think the basic
setup will remain.

Note this builds on #3414

toolchain/check/context.cpp

@@ -15,8 +15,10 @@
 #include "toolchain/lex/tokenized_buffer.h"
 #include "toolchain/parse/node_kind.h"
 #include "toolchain/sem_ir/file.h"
+#include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/inst.h"
 #include "toolchain/sem_ir/inst_kind.h"
+#include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Check {
 
@@ -112,6 +114,45 @@ auto Context::NoteIncompleteClass(SemIR::ClassId class_id,
   }
 }
 
+auto Context::AddPackageImports(Parse::Node import_node,
+                                IdentifierId package_id,
+                                llvm::ArrayRef<const SemIR::File*> sem_irs,
+                                bool has_load_error) -> void {
+  CARBON_CHECK(has_load_error || !sem_irs.empty())
+      << "There should be either a load error or at least one IR.";
+
+  auto name_id = SemIR::NameId::ForIdentifier(package_id);
+
+  SemIR::CrossRefIRId first_id(cross_ref_irs().size());
+  for (const auto* sem_ir : sem_irs) {
+    cross_ref_irs().Add(sem_ir);
+  }
+  if (has_load_error) {
+    cross_ref_irs().Add(nullptr);
+  }
+  SemIR::CrossRefIRId last_id(cross_ref_irs().size() - 1);
+
+  auto type_id = GetBuiltinType(SemIR::BuiltinKind::NamespaceType);
+  auto inst_id = AddInst(SemIR::Import{.parse_node = import_node,
+                                       .type_id = type_id,
+                                       .first_cross_ref_ir_id = first_id,
+                                       .last_cross_ref_ir_id = last_id});
+  if (name_id.is_valid()) {
+    // Add the import to lookup. Should always succeed because imports will be
+    // uniquely named.
+    AddNameToLookup(import_node, name_id, inst_id);
+    // Add a name for formatted output. This isn't used in name lookup in order
+    // to reduce indirection, but it's separate from the Import because it
+    // otherwise fits in an Inst.
+    AddInst(SemIR::BindName{.parse_node = import_node,
+                            .type_id = type_id,
+                            .name_id = name_id,
+                            .value_id = inst_id});
+  } else {
+    // TODO: All names from the current package should be added.
+  }
+}
+
 auto Context::AddNameToLookup(Parse::Node name_node, SemIR::NameId name_id,
                               SemIR::InstId target_id) -> void {
   if (current_scope().names.insert(name_id).second) {
@@ -811,6 +852,7 @@ class TypeCompleter {
       case SemIR::Deref::Kind:
       case SemIR::Field::Kind:
       case SemIR::FunctionDecl::Kind:
+      case SemIR::Import::Kind:
       case SemIR::InitializeFrom::Kind:
       case SemIR::IntegerLiteral::Kind:
       case SemIR::NameRef::Kind: