Move default value inst ids to a dedicated value store. (#7810)

Per #7737 we move the default value `InstId` storage from
a block in the `SemIR::Function` data structure to a
`SemIR::File` scoped `ValueStore`.

Moves the default value consistency checking to the general
merge argument pattern matching logic, which changes the
error message issued to the generic one.

toolchain/sem_ir/default_value.cpp

@@ -0,0 +1,12 @@
+// 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/default_value.h"
+
+#include "toolchain/base/value_store_impl.h"
+
+namespace Carbon {
+template class ValueStore<SemIR::DefaultValueId, SemIR::DefaultValue,
+                          Tag<SemIR::CheckIRId>>;
+}  // namespace Carbon