adds a field to SemIR::Interface to indicate whether it is a core interface (#7091)

The existing `GetCoreInterface` has linear-time complexity and adds more
than the search criteria to the `CoreInterfaceCache`. Adding a new field
to `Interface` changes this operation when building packages other than
`Core`, as this should only be set for the core library.

toolchain/sem_ir/core_interface.cpp

@@ -0,0 +1,15 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/sem_ir/core_interface.h"  // IWYU pragma: keep
+
+#include "toolchain/sem_ir/inst_kind.h"
+
+namespace Carbon::SemIR {
+CARBON_DEFINE_ENUM_CLASS_NAMES(CoreInterface) {
+#define CARBON_SEM_IR_CORE_INTERFACE_KIND(Name) \
+  CARBON_ENUM_CLASS_NAME_STRING(Name)
+#include "toolchain/sem_ir/core_interface_kind.def"
+};
+}  // namespace Carbon::SemIR