Add import context for locations. (#3807)

As discussed around #3792, identify the import a diagnostic message came
from prior to the diagnostic message itself. This occurs during location
translation so that the logic can be central.

I'd considered associating the parse node with ImportRef instructions,
but I realized about halfway through that because I need to store the
ImportDirectiveId on the ImportIR for cross-package imports, it's there
for use in location translation without extra work. That saves a fair
amount of stringing it through declarations, as well as an oddity where
ImportRef instructions would have a node that didn't really represent
them.

toolchain/check/import.cpp

@@ -130,12 +130,11 @@ static auto CacheCopiedNamespace(
 // name conflicts, but that won't change the result because namespaces supersede
 // other names in conflicts.
 static auto CopySingleNameScopeFromImportIR(
-    Context& context,
+    Context& context, SemIR::TypeId namespace_type_id,
     llvm::DenseMap<SemIR::NameScopeId, SemIR::NameScopeId>& copied_namespaces,
     SemIR::ImportIRId ir_id, SemIR::InstId import_inst_id,
     SemIR::NameScopeId import_scope_id, SemIR::NameScopeId enclosing_scope_id,
-    SemIR::NameId name_id, SemIR::TypeId namespace_type_id)
-    -> SemIR::NameScopeId {
+    SemIR::NameId name_id) -> SemIR::NameScopeId {
   // Produce the namespace for the entry.
   auto make_import_id = [&]() {
     return context.AddInst(SemIR::ImportRefUsed{.type_id = namespace_type_id,
@@ -199,8 +198,8 @@ static auto CopyEnclosingNameScopesFromImportIR(
     auto name_id =
         CopyNameFromImportIR(context, import_sem_ir, import_scope.name_id);
     scope_cursor = CopySingleNameScopeFromImportIR(
-        context, copied_namespaces, ir_id, import_scope.inst_id,
-        import_scope_id, scope_cursor, name_id, namespace_type_id);
+        context, namespace_type_id, copied_namespaces, ir_id,
+        import_scope.inst_id, import_scope_id, scope_cursor, name_id);
   }
 
   return scope_cursor;
@@ -208,8 +207,10 @@ static auto CopyEnclosingNameScopesFromImportIR(
 
 auto ImportLibraryFromCurrentPackage(Context& context,
                                      SemIR::TypeId namespace_type_id,
+                                     Parse::ImportDirectiveId node_id,
                                      const SemIR::File& import_sem_ir) -> void {
-  auto ir_id = context.import_irs().Add(&import_sem_ir);
+  auto ir_id =
+      context.import_irs().Add({.node_id = node_id, .sem_ir = &import_sem_ir});
   context.import_ir_constant_values()[ir_id.index].Set(
       SemIR::InstId::PackageNamespace,
       context.constant_values().Get(SemIR::InstId::PackageNamespace));
@@ -236,9 +237,8 @@ auto ImportLibraryFromCurrentPackage(Context& context,
       // Namespaces are always imported because they're essential for
       // qualifiers, and the type is simple.
       CopySingleNameScopeFromImportIR(
-          context, copied_namespaces, ir_id, import_inst_id,
-          import_namespace_inst->name_scope_id, enclosing_scope_id, name_id,
-          namespace_type_id);
+          context, namespace_type_id, copied_namespaces, ir_id, import_inst_id,
+          import_namespace_inst->name_scope_id, enclosing_scope_id, name_id);
     } else {
       // Leave a placeholder that the inst comes from the other IR.
       auto target_id = context.AddImportRef(ir_id, import_inst_id);
@@ -260,9 +260,9 @@ auto ImportLibrariesFromOtherPackage(Context& context,
                                      SemIR::TypeId namespace_type_id,
                                      Parse::ImportDirectiveId node_id,
                                      IdentifierId package_id,
-                                     llvm::ArrayRef<const SemIR::File*> sem_irs,
+                                     llvm::ArrayRef<SemIR::ImportIR> import_irs,
                                      bool has_load_error) -> void {
-  CARBON_CHECK(has_load_error || !sem_irs.empty())
+  CARBON_CHECK(has_load_error || !import_irs.empty())
       << "There should be either a load error or at least one IR.";
 
   auto name_id = SemIR::NameId::ForIdentifier(package_id);
@@ -273,8 +273,8 @@ auto ImportLibrariesFromOtherPackage(Context& context,
 
   auto& scope = context.name_scopes().Get(namespace_scope_id);
   scope.is_closed_import = !is_duplicate;
-  for (const auto* sem_ir : sem_irs) {
-    auto ir_id = context.import_irs().Add(sem_ir);
+  for (auto import_ir : import_irs) {
+    auto ir_id = context.import_irs().Add(import_ir);
     scope.import_ir_scopes.push_back({ir_id, SemIR::NameScopeId::Package});
     context.import_ir_constant_values()[ir_id.index].Set(
         SemIR::InstId::PackageNamespace, namespace_const_id);