Remove ImportRefUsed (#3934)

This was to track use of a declaration after import, prior to a
redeclaration. Per [discussion on
Discord](https://discord.com/channels/655572317891461132/1217182321933815820/1236016521059237962),
we likely don't need this check due to the change in behavior of
`extern`.

Rather than potentially getting one of many `extern` decls and depending
on it by accident, it is now planned to be _required_ to be imported,
and the library doing a non-`extern` decl must _know_ it's importing the
`extern` decl. The stricter requirement on the library means it now
seems more reasonable to use the `extern` decl.

So kind of rolling back #3831, though keeping `ImportIRInstId` (at least
for now) and keeping `Loaded`/`Unloaded` terminology (seems a nicer
fit).

toolchain/check/context.cpp

@@ -211,8 +211,7 @@ auto Context::AddNameToLookup(SemIR::NameId name_id, SemIR::InstId target_id)
 }
 
 auto Context::LookupNameInDecl(SemIR::LocId loc_id, SemIR::NameId name_id,
-                               SemIR::NameScopeId scope_id,
-                               bool mark_imports_used) -> SemIR::InstId {
+                               SemIR::NameScopeId scope_id) -> SemIR::InstId {
   if (!scope_id.is_valid()) {
     // Look for a name in the current scope only. There are two cases where the
     // name would be in an outer scope:
@@ -248,8 +247,7 @@ auto Context::LookupNameInDecl(SemIR::LocId loc_id, SemIR::NameId name_id,
     //
     //    // Error, no `F` in `B`.
     //    fn B.F() {}
-    return LookupNameInExactScope(loc_id, name_id, name_scopes().Get(scope_id),
-                                  mark_imports_used);
+    return LookupNameInExactScope(loc_id, name_id, name_scopes().Get(scope_id));
   }
 }
 
@@ -284,8 +282,8 @@ auto Context::LookupUnqualifiedName(Parse::NodeId node_id,
 // Handles lookup through the import_ir_scopes for LookupNameInExactScope.
 static auto LookupInImportIRScopes(Context& context, SemIRLoc loc,
                                    SemIR::NameId name_id,
-                                   const SemIR::NameScope& scope,
-                                   bool mark_imports_used) -> SemIR::InstId {
+                                   const SemIR::NameScope& scope)
+    -> SemIR::InstId {
   auto identifier_id = name_id.AsIdentifierId();
   llvm::StringRef identifier;
   if (identifier_id.is_valid()) {
@@ -328,8 +326,7 @@ static auto LookupInImportIRScopes(Context& context, SemIRLoc loc,
     if (result_id.is_valid()) {
       context.DiagnoseDuplicateName(import_inst_id, result_id);
     } else {
-      LoadImportRef(context, import_inst_id,
-                    mark_imports_used ? loc : SemIR::LocId::Invalid);
+      LoadImportRef(context, import_inst_id);
       result_id = import_inst_id;
     }
   }
@@ -338,16 +335,14 @@ static auto LookupInImportIRScopes(Context& context, SemIRLoc loc,
 }
 
 auto Context::LookupNameInExactScope(SemIRLoc loc, SemIR::NameId name_id,
-                                     const SemIR::NameScope& scope,
-                                     bool mark_imports_used) -> SemIR::InstId {
+                                     const SemIR::NameScope& scope)
+    -> SemIR::InstId {
   if (auto it = scope.names.find(name_id); it != scope.names.end()) {
-    LoadImportRef(*this, it->second,
-                  mark_imports_used ? loc : SemIR::LocId::Invalid);
+    LoadImportRef(*this, it->second);
     return it->second;
   }
   if (!scope.import_ir_scopes.empty()) {
-    return LookupInImportIRScopes(*this, loc, name_id, scope,
-                                  mark_imports_used);
+    return LookupInImportIRScopes(*this, loc, name_id, scope);
   }
   return SemIR::InstId::Invalid;
 }
@@ -364,8 +359,7 @@ auto Context::LookupQualifiedName(Parse::NodeId node_id, SemIR::NameId name_id,
     const auto& scope = name_scopes().Get(scope_ids.pop_back_val());
     has_error |= scope.has_error;
 
-    auto scope_result_id = LookupNameInExactScope(node_id, name_id, scope,
-                                                  /*mark_imports_used=*/true);
+    auto scope_result_id = LookupNameInExactScope(node_id, name_id, scope);
     if (!scope_result_id.is_valid()) {
       // Nothing found in this scope: also look in its extended scopes.
       auto extended = llvm::reverse(scope.extended_scopes);
@@ -415,8 +409,8 @@ static auto GetCorePackage(Context& context, SemIRLoc loc)
 
   // Look up `package.Core`.
   auto core_inst_id = context.LookupNameInExactScope(
-      loc, core_name_id, context.name_scopes().Get(SemIR::NameScopeId::Package),
-      /*mark_imports_used=*/true);
+      loc, core_name_id,
+      context.name_scopes().Get(SemIR::NameScopeId::Package));
   if (!core_inst_id.is_valid()) {
     context.DiagnoseNameNotFound(loc, core_name_id);
     return SemIR::NameScopeId::Invalid;
@@ -441,8 +435,7 @@ auto Context::LookupNameInCore(SemIRLoc loc, llvm::StringRef name)
 
   auto name_id = SemIR::NameId::ForIdentifier(identifiers().Add(name));
   auto inst_id =
-      LookupNameInExactScope(loc, name_id, name_scopes().Get(core_package_id),
-                             /*mark_imports_used=*/true);
+      LookupNameInExactScope(loc, name_id, name_scopes().Get(core_package_id));
   if (!inst_id.is_valid()) {
     DiagnoseNameNotFound(loc, name_id);
     return SemIR::InstId::BuiltinError;